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- Herausgeber: libreka classics
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
- Veröffentlichungsjahr: 2019
A History of Science - Volume 4. Modern development of the chemical and biological sciences libreka classics – These are classics of literary history, reissued and made available to a wide audience. Immerse yourself in well-known and popular titles!
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Titel: A History of Science — Volume 4
von William Shakespeare, H. G. Wells, Henry Van Dyke, Thomas Carlyle, Oscar Wilde, Joseph Conrad, Henry James, Anthony Hope, Henry Fielding, Giraldus Cambrensis, Daniel Defoe, Grammaticus Saxo, Edgar Rice Burroughs, Hugh Lofting, Agatha Christie, Sinclair Lewis, Eugène Brieux, Upton Sinclair, Booth Tarkington, Sax Rohmer, Jack London, Anna Katharine Green, Sara Jeannette Duncan, Xenophon, Alexandre Dumas père, John William Draper, Alice Christiana Thompson Meynell, Bram Stoker, Honoré de Balzac, William Congreve, Louis de Rougemont, Nikolai Vasilievich Gogol, Rolf Boldrewood, François Rabelais, Lysander Spooner, B. M. Bower, Henry Rider Haggard, William Hickling Prescott, Lafcadio Hearn, Robert Herrick, Jane Austen, Mark Twain, Mary Roberts Rinehart, Charles Babbage, Kate Douglas Smith Wiggin, Frank L. Packard, George Meredith, John Merle Coulter, Irvin S. Cobb, Edwin Mims, John Tyndall, Various, Charles Darwin, Sidney Lanier, Henry Lawson, Niccolò Machiavelli, George W. Crile, Théophile Gautier, Noah Brooks, James Thomson, Zane Grey, J. M. Synge, Virginia Woolf, Conrad Aiken, Edna St. Vincent Millay, Helen Cody Wetmore, Ayn Rand, Sir Thomas Malory, Gustave Flaubert, Edmond Rostand, Charlotte Brontë, Edith Wharton, Giles Lytton Strachey, Myrtle Reed, Ernest Bramah, Jules Verne, H. L. Mencken, H. Stanley Redgrove, Victor Lefebure, Edna Lyall, John Masefield, Charles Kingsley, Robert Burns, Edgar Lee Masters, Victor [pseud.] Appleton, Ellis Parker Butler, Mary Lamb, Charles Lamb, Johann Wolfgang von Goethe, Kenneth Grahame, Charles Dickens, John Ruskin, John Galt, James J. Davis, Owen Wister, William Blades, Sir Hall Caine, Sir Max Beerbohm, Baron Edward John Moreton Drax Plunkett Dunsany, Bret Harte, E. Phillips Oppenheim, Thomas Henry Huxley, A. B. Paterson, John N. Reynolds, Walter Dill Scott, Hans Gustav Adolf Gross, T. S. Eliot, Walt Whitman, Arthur Ransome, Jane Addams, Elizabeth, David Lindsay, Helen Bannerman, Charles A. Oliver, J. M. Barrie, Robert F. Murray, Andrew Lang, Jerome K. Jerome, Francis Thompson, Sydney Waterlow, Andrew Dickson White, Benjamin N. Cardozo, Karl Marx, Edouard Louis Emmanuel Julien Le Roy, Margaret Hill McCarter, Sir Donald Mackenzie Wallace, Howard Trueman, L. M. Montgomery, Frank T. Bullen, Baron Alfred Tennyson Tennyson, Jonathan Nield, Henry Wadsworth Longfellow, Charles Reade, Ouida, Washington Irving, Benjamin Louis Eulalie de Bonneville, Sir Walter Scott, Stewart Edward White, Arthur Hugh Clough, Baron Edward Bulwer Lytton Lytton, C.-F. Volney, T. Troward, graf Leo Tolstoy, Christopher Morley, James Madison, Alexander Hamilton, John Jay, Gilbert White, Percival Lowell, Frederick Marryat, Robert Graves, Thomas Holmes, Wilkie Collins, Maria Edgeworth, Katherine Mansfield, E. Nesbit, Olive Schreiner, Jeronimo Lobo, O. Henry, James Slough Zerbe, Donald Ogden Stewart, Johanna Spyri, Eleanor H. Porter, William Tatem Tilden, Sol Plaatje, Rafael Sabatini, William Makepeace Thackeray, George Gissing, Maksim Gorky, Baron Thomas Babington Macaulay Macaulay, H. G. Keene, Saki, R. B. Cunninghame Graham, Thomas Hughes, David Nunes Carvalho, Vicente Blasco Ibáñez, Carry Amelia Nation, John Fiske, Bernard Shaw, Elbridge Streeter Brooks, William Holmes McGuffey, Edward Everett Hale, Louis Ginzberg, Chester K. Steele, Christopher Marlowe, Plato, John Lord, Shakespeare, Martin Luther, Frances Hodgson Burnett, Howard Pyle, Charles Morris, Edward Carpenter, Maurice Leblanc, James Boswell, William Osler, William Ernest Henley, Theron Q. Dumont, Horatio Alger, Abraham Myerson, Joel Benton, Eden Phillpotts, Anonymous, Robert Louis Stevenson, Lloyd Osbourne, Cleland Boyd McAfee, Robert Williams Wood, H. C. Andersen, Edna Ferber, James Stephens, John Jacob Astor, Alexandre Dumas fils, Hilda Conkling, J. Storer Clouston, Julian Hawthorne, Ernest Albert Savage, Mary Eleanor Wilkins Freeman, Fernando de Rojas, Richard Harding Davis, Charles Whibley, Thomas Dixon, Sir Arthur Conan Doyle, George MacDonald, Thomas H. Burgoyne, Belle M. Wagner, Émile Gaboriau, à Kempis Thomas, United States. Central Intelligence Agency, Herbert Darling Foster, John Chipman Farrar, Lucius Apuleius, Olive Gilbert, Sojourner Truth, Arthur Judson Brown, Burbank L. Todd, Gaston Leroux, Margaret Sanger, Jr. Martin Luther King, Mary Johnston, S. A. Reilly, G. K. Chesterton, Elizabeth Cleghorn Gaskell, George Iles, E. W. Hornung, Edward Huntington Williams, Henry Smith Williams
ISBN 978-3-7429-1650-1
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A HISTORY OF SCIENCE
Contents
I. THE PHLOGISTON THEORY IN CHEMISTRY
II. THE BEGINNINGS OF MODERN CHEMISTRY
III. CHEMISTRY SINCE THE TIME OF DALTON
IV. ANATOMY AND PHYSIOLOGY IN THE EIGHTEENTH CENTURY
V. ANATOMY AND PHYSIOLOGY IN THE NINETEENTH CENTURY
VI. THEORIES OF ORGANIC EVOLUTION
VII. EIGHTEENTH-CENTURY MEDICINE
VIII. NINETEENTH-CENTURY MEDICINE
IX. THE NEW SCIENCE OF EXPERIMENTAL PSYCHOLOGY
X. THE NEW SCIENCE OF ORIENTAL ARCHAEOLOGY
APPENDIX
BOOK IV. MODERN DEVELOPMENT OF THE CHEMICAL AND BIOLOGICAL SCIENCES
AS regards chronology, the epoch covered in the present volume is identical with that viewed in the preceding one. But now as regards subject matter we pass on to those diverse phases of the physical world which are the field of the chemist, and to those yet more intricate processes which have to do with living organisms. So radical are the changes here that we seem to be entering new worlds; and yet, here as before, there are intimations of the new discoveries away back in the Greek days. The solution of the problem of respiration will remind us that Anaxagoras half guessed the secret; and in those diversified studies which tell us of the Daltonian atom in its wonderful transmutations, we shall be reminded again of the Clazomenian philosopher and his successor Democritus.
Yet we should press the analogy much too far were we to intimate that the Greek of the elder day or any thinker of a more recent period had penetrated, even in the vaguest way, all of the mysteries that the nineteenth century has revealed in the fields of chemistry and biology. At the very most the insight of those great Greeks and of the wonderful seventeenth-century philosophers who so often seemed on the verge of our later discoveries did no more than vaguely anticipate their successors of this later century. To gain an accurate, really specific knowledge of the properties of elementary bodies was reserved for the chemists of a recent epoch. The vague Greek questionings as to organic evolution were world-wide from the precise inductions of a Darwin. If the mediaeval Arabian endeavored to dull the knife of the surgeon with the use of drugs, his results hardly merit to be termed even an anticipation of modern anaesthesia. And when we speak of preventive medicine—of bacteriology in all its phases—we have to do with a marvellous field of which no previous generation of men had even the slightest inkling.
All in all, then, those that lie before us are perhaps the most wonderful and the most fascinating of all the fields of science. As the chapters of the preceding book carried us out into a macrocosm of inconceivable magnitude, our present studies are to reveal a microcosm of equally inconceivable smallness. As the studies of the physicist attempted to reveal the very nature of matter and of energy, we have now to seek the solution of the yet more inscrutable problems of life and of mind.
I. THE PHLOGISTON THEORY IN CHEMISTRY
The development of the science of chemistry from the "science" of alchemy is a striking example of the complete revolution in the attitude of observers in the field of science. As has been pointed out in a preceding chapter, the alchemist, having a preconceived idea of how things should be, made all his experiments to prove his preconceived theory; while the chemist reverses this attitude of mind and bases his conceptions on the results of his laboratory experiments. In short, chemistry is what alchemy never could be, an inductive science. But this transition from one point of view to an exactly opposite one was necessarily a very slow process. Ideas that have held undisputed sway over the minds of succeeding generations for hundreds of years cannot be overthrown in a moment, unless the agent of such an overthrow be so obvious that it cannot be challenged. The rudimentary chemistry that overthrew alchemy had nothing so obvious and palpable.
The great first step was the substitution of the one principle, phlogiston, for the three principles, salt, sulphur, and mercury. We have seen how the experiment of burning or calcining such a metal as lead "destroyed" the lead as such, leaving an entirely different substance in its place, and how the original metal could be restored by the addition of wheat to the calcined product. To the alchemist this was "mortification" and "revivification" of the metal. For, as pointed out by Paracelsus, "anything that could be killed by man could also be revivified by him, although this was not possible to the things killed by God." The burning of such substances as wood, wax, oil, etc., was also looked upon as the same "killing" process, and the fact that the alchemist was unable to revivify them was regarded as simply the lack of skill on his part, and in no wise affecting the theory itself.
But the iconoclastic spirit, if not the acceptance of all the teachings, of the great Paracelsus had been gradually taking root among the better class of alchemists, and about the middle of the seventeenth century Robert Boyle (1626-1691) called attention to the possibility of making a wrong deduction from the phenomenon of the calcination of the metals, because of a very important factor, the action of the air, which was generally overlooked. And he urged his colleagues of the laboratories to give greater heed to certain other phenomena that might pass unnoticed in the ordinary calcinating process. In his work, The Sceptical Chemist, he showed the reasons for doubting the threefold constitution of matter; and in his General History of the Air advanced some novel and carefully studied theories as to the composition of the atmosphere. This was an important step, and although Boyle is not directly responsible for the phlogiston theory, it is probable that his experiments on the atmosphere influenced considerably the real founders, Becker and Stahl.
Boyle gave very definitely his idea of how he thought air might be composed. "I conjecture that the atmospherical air consists of three different kinds of corpuscles," he says; "the first, those numberless particles which, in the form of vapors or dry exhalations, ascend from the earth, water, minerals, vegetables, animals, etc.; in a word, whatever substances are elevated by the celestial or subterraneal heat, and thence diffused into the atmosphere. The second may be yet more subtle, and consist of those exceedingly minute atoms, the magnetical effluvia of the earth, with other innumerable particles sent out from the bodies of the celestial luminaries, and causing, by their influence, the idea of light in us. The third sort is its characteristic and essential property, I mean permanently elastic parts. Various hypotheses may be framed relating to the structure of these later particles of the air. They might be resembled to the springs of watches, coiled up and endeavoring to restore themselves; to wool, which, being compressed, has an elastic force; to slender wires of different substances, consistencies, lengths, and thickness; in greater curls or less, near to, or remote from each other, etc., yet all continuing springy, expansible, and compressible. Lastly, they may also be compared to the thin shavings of different kinds of wood, various in their lengths, breadth, and thickness. And this, perhaps, will seem the most eligible hypothesis, because it, in some measure, illustrates the production of the elastic particles we are considering. For no art or curious instruments are required to make these shavings whose curls are in no wise uniform, but seemingly casual; and what is more remarkable, bodies that before seemed unelastic, as beams and blocks, will afford them."(1)
Although this explanation of the composition of the air is most crude, it had the effect of directing attention to the fact that the atmosphere is not "mere nothingness," but a "something" with a definite composition, and this served as a good foundation for future investigations. To be sure, Boyle was neither the first nor the only chemist who had suspected that the air was a mixture of gases, and not a simple one, and that only certain of these gases take part in the process of calcination. Jean Rey, a French physician, and John Mayow, an Englishman, had preformed experiments which showed conclusively that the air was not a simple substance; but Boyle's work was better known, and in its effect probably more important. But with all Boyle's explanations of the composition of air, he still believed that there was an inexplicable something, a "vital substance," which he was unable to fathom, and which later became the basis of Stahl's phlogiston theory. Commenting on this mysterious substance, Boyle says: "The difficulty we find in keeping flame and fire alive, though but for a little time, without air, renders it suspicious that there be dispersed through the rest of the atmosphere some odd substance, either of a solar, astral, or other foreign nature; on account of which the air is so necessary to the substance of flame!" It was this idea that attracted the attention of George Ernst Stahl (1660-1734), a professor of medicine in the University of Halle, who later founded his new theory upon it. Stahl's theory was a development of an earlier chemist, Johann Joachim Becker (1635-1682), in whose footsteps he followed and whose experiments he carried further.
In many experiments Stahl had been struck with the fact that certain substances, while differing widely, from one another in many respects, were alike in combustibility. From this he argued that all combustible substances must contain a common principle, and this principle he named phlogiston. This phlogiston he believed to be intimately associated in combination with other substances in nature, and in that condition not perceivable by the senses; but it was supposed to escape as a substance burned, and become apparent to the senses as fire or flame. In other words, phlogiston was something imprisoned in a combustible structure (itself forming part of the structure), and only liberated when this structure was destroyed. Fire, or flame, was FREE phlogiston, while the imprisoned phlogiston was called COMBINED PHLOGISTON, or combined fire. The peculiar quality of this strange substance was that it disliked freedom and was always striving to conceal itself in some combustible substance. Boyle's tentative suggestion that heat was simply motion was apparently not accepted by Stahl, or perhaps it was unknown to him.
According to the phlogistic theory, the part remaining after a substance was burned was simply the original substance deprived of phlogiston. To restore the original combustible substance, it was necessary to heat the residue of the combustion with something that burned easily, so that the freed phlogiston might again combine with the ashes. This was explained by the supposition that the more combustible a substance was the more phlogiston it contained, and since free phlogiston sought always to combine with some suitable substance, it was only necessary to mix the phlogisticating agents, such as charcoal, phosphorus, oils, fats, etc., with the ashes of the original substance, and heat the mixture, the phlogiston thus freed uniting at once with the ashes. This theory fitted very nicely as applied to the calcined lead revivified by the grains of wheat, although with some other products of calcination it did not seem to apply at all.
It will be seen from this that the phlogistic theory was a step towards chemistry and away from alchemy. It led away from the idea of a "spirit" in metals that could not be seen, felt, or appreciated by any of the senses, and substituted for it a principle which, although a falsely conceived one, was still much more tangible than the "spirit," since it could be seen and felt as free phlogiston and weighed and measured as combined phlogiston. The definiteness of the statement that a metal, for example, was composed of phlogiston and an element was much less enigmatic, even if wrong, than the statement of the alchemist that "metals are produced by the spiritual action of the three principles, salt, mercury, sulphur"—particularly when it is explained that salt, mercury, and sulphur were really not what their names implied, and that there was no universally accepted belief as to what they really were.
The metals, which are now regarded as elementary bodies, were considered compounds by the phlogistians, and they believed that the calcining of a metal was a process of simplification. They noted, however, that the remains of calcination weighed more than the original product, and the natural inference from this would be that the metal must have taken in some substance rather than have given off anything. But the phlogistians had not learned the all-important significance of weights, and their explanation of variation in weight was either that such gain or loss was an unimportant "accident" at best, or that phlogiston, being light, tended to lighten any substance containing it, so that driving it out of the metal by calcination naturally left the residue heavier.
At first the phlogiston theory seemed to explain in an indisputable way all the known chemical phenomena. Gradually, however, as experiments multiplied, it became evident that the plain theory as stated by Stahl and his followers failed to explain satisfactorily certain laboratory reactions. To meet these new conditions, certain modifications were introduced from time to time, giving the theory a flexibility that would allow it to cover all cases. But as the number of inexplicable experiments continued to increase, and new modifications to the theory became necessary, it was found that some of these modifications were directly contradictory to others, and thus the simple theory became too cumbersome from the number of its modifications. Its supporters disagreed among themselves, first as to the explanation of certain phenomena that did not seem to accord with the phlogistic theory, and a little later as to the theory itself. But as yet there was no satisfactory substitute for this theory, which, even if unsatisfactory, seemed better than anything that had gone before or could be suggested.
But the good effects of the era of experimental research, to which the theory of Stahl had given such an impetus, were showing in the attitude of the experimenters. The works of some of the older writers, such as Boyle and Hooke, were again sought out in their dusty corners and consulted, and their surmises as to the possible mixture of various gases in the air were more carefully considered. Still the phlogiston theory was firmly grounded in the minds of the philosophers, who can hardly be censured for adhering to it, at least until some satisfactory substitute was offered. The foundation for such a theory was finally laid, as we shall see presently, by the work of Black, Priestley, Cavendish, and Lavoisier, in the eighteenth century, but the phlogiston theory cannot be said to have finally succumbed until the opening years of the nineteenth century.
II. THE BEGINNINGS OF MODERN CHEMISTRY
THE "PNEUMATIC" CHEMISTS
Modern chemistry may be said to have its beginning with the work of Stephen Hales (1677-1761), who early in the eighteenth century began his important study of the elasticity of air. Departing from the point of view of most of the scientists of the time, he considered air to be "a fine elastic fluid, with particles of very different nature floating in it"; and he showed that these "particles" could be separated. He pointed out, also, that various gases, or "airs," as he called them, were contained in many solid substances. The importance of his work, however, lies in the fact that his general studies were along lines leading away from the accepted doctrines of the time, and that they gave the impetus to the investigation of the properties of gases by such chemists as Black, Priestley, Cavendish, and Lavoisier, whose specific discoveries are the foundation-stones of modern chemistry.
The careful studies of Hales were continued by his younger confrere, Dr. Joseph Black (1728-1799), whose experiments in the weights of gases and other chemicals were first steps in quantitative chemistry. But even more important than his discoveries of chemical properties in general was his discovery of the properties of carbonic-acid gas.
Black had been educated for the medical profession in the University of Glasgow, being a friend and pupil of the famous Dr. William Cullen. But his liking was for the chemical laboratory rather than for the practice of medicine. Within three years after completing his medical course, and when only twenty-three years of age, he made the discovery of the properties of carbonic acid, which he called by the name of "fixed air." After discovering this gas, Black made a long series of experiments, by which he was able to show how widely it was distributed throughout nature. Thus, in 1757, he discovered that the bubbles given off in the process of brewing, where there was vegetable fermentation, were composed of it. To prove this, he collected the contents of these bubbles in a bottle containing lime-water. When this bottle was shaken violently, so that the lime-water and the carbonic acid became thoroughly mixed, an insoluble white powder was precipitated from the solution, the carbonic acid having combined chemically with the lime to form the insoluble calcium carbonate, or chalk. This experiment suggested another. Fixing a piece of burning charcoal in the end of a bellows, he arranged a tube so that the gas coming from the charcoal would pass through the lime-water, and, as in the case of the bubbles from the brewer's vat, he found that the white precipitate was thrown down; in short, that carbonic acid was given off in combustion. Shortly after, Black discovered that by blowing through a glass tube inserted into lime-water, chalk was precipitated, thus proving that carbonic acid was being constantly thrown off in respiration.
The effect of Black's discoveries was revolutionary, and the attitude of mind of the chemists towards gases, or "airs," was changed from that time forward. Most of the chemists, however, attempted to harmonize the new facts with the older theories—to explain all the phenomena on the basis of the phlogiston theory, which was still dominant. But while many of Black's discoveries could not be made to harmonize with that theory, they did not directly overthrow it. It required the additional discoveries of some of Black's fellow-scientists to complete its downfall, as we shall see.
This work of Black's was followed by the equally important work of his former pupil, Henry Cavendish (1731-1810), whose discovery of the composition of many substances, notably of nitric acid and of water, was of great importance, adding another link to the important chain of evidence against the phlogiston theory. Cavendish is one of the most eccentric figures in the history of science, being widely known in his own time for his immense wealth and brilliant intellect, and also for his peculiarities and his morbid sensibility, which made him dread society, and probably did much in determining his career. Fortunately for him, and incidentally for the cause of science, he was able to pursue laboratory investigations without being obliged to mingle with his dreaded fellow-mortals, his every want being provided for by the immense fortune inherited from his father and an uncle.
When a young man, as a pupil of Dr. Black, he had become imbued with the enthusiasm of his teacher, continuing Black's investigations as to the properties of carbonic-acid gas when free and in combination. One of his first investigations was reported in 1766, when he communicated to the Royal Society his experiments for ascertaining the properties of carbonic-acid and hydrogen gas, in which he first showed the possibility of weighing permanently elastic fluids, although Torricelli had before this shown the relative weights of a column of air and a column of mercury. Other important experiments were continued by Cavendish, and in 1784 he announced his discovery of the composition of water, thus robbing it of its time-honored position as an "element." But his claim to priority in this discovery was at once disputed by his fellow-countryman James Watt and by the Frenchman Lavoisier. Lavoisier's claim was soon disallowed even by his own countrymen, but for many years a bitter controversy was carried on by the partisans of Watt and Cavendish. The two principals, however, seem never to have entered into this controversy with anything like the same ardor as some of their successors, as they remained on the best of terms.(1) It is certain, at any rate, that Cavendish announced his discovery officially before Watt claimed that the announcement had been previously made by him, "and, whether right or wrong, the honor of scientific discoveries seems to be accorded naturally to the man who first publishes a demonstration of his discovery." Englishmen very generally admit the justness of Cavendish's claim, although the French scientist Arago, after reviewing the evidence carefully in 1833, decided in favor of Watt.
It appears that something like a year before Cavendish made known his complete demonstration of the composition of water, Watt communicated to the Royal Society a suggestion that water was composed of "dephlogisticated air (oxygen) and phlogiston (hydrogen) deprived of part of its latent heat." Cavendish knew of the suggestion, but in his experiments refuted the idea that the hydrogen lost any of its latent heat. Furthermore, Watt merely suggested the possible composition without proving it, although his idea was practically correct, if we can rightly interpret the vagaries of the nomenclature then in use. But had Watt taken the steps to demonstrate his theory, the great "Water Controversy" would have been avoided. Cavendish's report of his discovery to the Royal Society covers something like forty pages of printed matter. In this he shows how, by passing an electric spark through a closed jar containing a mixture of hydrogen gas and oxygen, water is invariably formed, apparently by the union of the two gases. The experiment was first tried with hydrogen and common air, the oxygen of the air uniting with the hydrogen to form water, leaving the nitrogen of the air still to be accounted for. With pure oxygen and hydrogen, however, Cavendish found that pure water was formed, leaving slight traces of any other, substance which might not be interpreted as being Chemical impurities. There was only one possible explanation of this phenomenon—that hydrogen and oxygen, when combined, form water.
"By experiments with the globe it appeared," wrote Cavendish, "that when inflammable and common air are exploded in a proper proportion, almost all the inflammable air, and near one-fifth the common air, lose their elasticity and are condensed into dew. And by this experiment it appears that this dew is plain water, and consequently that almost all the inflammable air is turned into pure water.
"In order to examine the nature of the matter condensed on firing a mixture of dephlogisticated and inflammable air, I took a glass globe, holding 8800 grain measures, furnished with a brass cock and an apparatus for firing by electricity. This globe was well exhausted by an air-pump, and then filled with a mixture of inflammable and dephlogisticated air by shutting the cock, fastening the bent glass tube into its mouth, and letting up the end of it into a glass jar inverted into water and containing a mixture of 19,500 grain measures of dephlogisticated air, and 37,000 of inflammable air; so that, upon opening the cock, some of this mixed air rushed through the bent tube and filled the globe. The cock was then shut and the included air fired by electricity, by means of which almost all of it lost its elasticity (was condensed into water vapors). The cock was then again opened so as to let in more of the same air to supply the place of that destroyed by the explosion, which was again fired, and the operation continued till almost the whole of the mixture was let into the globe and exploded. By this means, though the globe held not more than a sixth part of the mixture, almost the whole of it was exploded therein without any fresh exhaustion of the globe."
At first this condensed matter was "acid to the taste and contained two grains of nitre," but Cavendish, suspecting that this was due to impurities, tried another experiment that proved conclusively that his opinions were correct. "I therefore made another experiment," he says, "with some more of the same air from plants in which the proportion of inflammable air was greater, so that the burnt air was almost completely phlogisticated, its standard being one-tenth. The condensed liquor was then not at all acid, but seemed pure water."
From these experiments he concludes "that when a mixture of inflammable and dephlogisticated air is exploded, in such proportions that the burnt air is not much phlogisticated, the condensed liquor contains a little acid which is always of the nitrous kind, whatever substance the dephlogisticated air is procured from; but if the proportion be such that the burnt air is almost entirely phlogisticated, the condensed liquor is not at all acid, but seems pure water, without any addition whatever."(2)
These same experiments, which were undertaken to discover the composition of water, led him to discover also the composition of nitric acid. He had observed that, in the combustion of hydrogen gas with common air, the water was slightly tinged with acid, but that this was not the case when pure oxygen gas was used. Acting upon this observation, he devised an experiment to determine the nature of this acid. He constructed an apparatus whereby an electric spark was passed through a vessel containing common air. After this process had been carried on for several weeks a small amount of liquid was formed. This liquid combined with a solution of potash to form common nitre, which "detonated with charcoal, sparkled when paper impregnated with it was burned, and gave out nitrous fumes when sulphuric acid was poured on it." In other words, the liquid was shown to be nitric acid. Now, since nothing but pure air had been used in the initial experiment, and since air is composed of nitrogen and oxygen, there seemed no room to doubt that nitric acid is a combination of nitrogen and oxygen.
This discovery of the nature of nitric acid seems to have been about the last work of importance that Cavendish did in the field of chemistry, although almost to the hour of his death he was constantly occupied with scientific observations. Even in the last moments of his life this habit asserted itself, according to Lord Brougham. "He died on March 10, 1810, after a short illness, probably the first, as well as the last, which he ever suffered. His habit of curious observation continued to the end. He was desirous of marking the progress of the disease and the gradual extinction of the vital powers. With these ends in view, that he might not be disturbed, he desired to be left alone. His servant, returning sooner than he had wished, was ordered again to leave the chamber of death, and when he came back a second time he found his master had expired."(3)
While the opulent but diffident Cavendish was making his important discoveries, another Englishman, a poor country preacher named Joseph Priestley (1733-1804) was not only rivalling him, but, if anything, outstripping him in the pursuit of chemical discoveries. In 1761 this young minister was given a position as tutor in a nonconformist academy at Warrington, and here, for six years, he was able to pursue his studies in chemistry and electricity. In 1766, while on a visit to London, he met Benjamin Franklin, at whose suggestion he published his History of Electricity. From this time on he made steady progress in scientific investigations, keeping up his ecclesiastical duties at the same time. In 1780 he removed to Birmingham, having there for associates such scientists as James Watt, Boulton, and Erasmus Darwin.
Eleven years later, on the anniversary of the fall of the Bastile in Paris, a fanatical mob, knowing Priestley's sympathies with the French revolutionists, attacked his house and chapel, burning both and destroying a great number of valuable papers and scientific instruments. Priestley and his family escaped violence by flight, but his most cherished possessions were destroyed; and three years later he quitted England forever, removing to the United States, whose struggle for liberty he had championed. The last ten years of his life were spent at Northumberland, Pennsylvania, where he continued his scientific researches.
Early in his scientific career Priestley began investigations upon the "fixed air" of Dr. Black, and, oddly enough, he was stimulated to this by the same thing that had influenced Black—that is, his residence in the immediate neighborhood of a brewery. It was during the course of a series of experiments on this and other gases that he made his greatest discovery, that of oxygen, or "dephlogisticated air," as he called it. The story of this important discovery is probably best told in Priestley's own words:
"There are, I believe, very few maxims in philosophy that have laid firmer hold upon the mind than that air, meaning atmospheric air, is a simple elementary substance, indestructible and unalterable, at least as much so as water is supposed to be. In the course of my inquiries I was, however, soon satisfied that atmospheric air is not an unalterable thing; for that, according to my first hypothesis, the phlogiston with which it becomes loaded from bodies burning in it, and the animals breathing it, and various other chemical processes, so far alters and depraves it as to render it altogether unfit for inflammation, respiration, and other purposes to which it is subservient; and I had discovered that agitation in the water, the process of vegetation, and probably other natural processes, restore it to its original purity....
"Having procured a lens of twelve inches diameter and twenty inches local distance, I proceeded with the greatest alacrity, by the help of it, to discover what kind of air a great variety of substances would yield, putting them into the vessel, which I filled with quicksilver, and kept inverted in a basin of the same .... With this apparatus, after a variety of experiments.... on the 1st of August, 1774, I endeavored to extract air from mercurius calcinatus per se; and I presently found that, by means of this lens, air was expelled from it very readily. Having got about three or four times as much as the bulk of my materials, I admitted water to it, and found that it was not imbibed by it. But what surprised me more than I can express was that a candle burned in this air with a remarkably vigorous flame, very much like that enlarged flame with which a candle burns in nitrous oxide, exposed to iron or liver of sulphur; but as I had got nothing like this remarkable appearance from any kind of air besides this particular modification of vitrous air, and I knew no vitrous acid was used in the preparation of mercurius calcinatus, I was utterly at a loss to account for it."(4)
The "new air" was, of course, oxygen. Priestley at once proceeded to examine it by a long series of careful experiments, in which, as will be seen, he discovered most of the remarkable qualities of this gas. Continuing his description of these experiments, he says:
"The flame of the candle, besides being larger, burned with more splendor and heat than in that species of nitrous air; and a piece of red-hot wood sparkled in it, exactly like paper dipped in a solution of nitre, and it consumed very fast; an experiment that I had never thought of trying with dephlogisticated nitrous air.
"... I had so little suspicion of the air from the mercurius calcinatus, etc., being wholesome, that I had not even thought of applying it to the test of nitrous air; but thinking (as my reader must imagine I frequently must have done) on the candle burning in it after long agitation in water, it occurred to me at last to make the experiment; and, putting one measure of nitrous air to two measures of this air, I found not only that it was diminished, but that it was diminished quite as much as common air, and that the redness of the mixture was likewise equal to a similar mixture of nitrous and common air.... The next day I was more surprised than ever I had been before with finding that, after the above-mentioned mixture of nitrous air and the air from mercurius calcinatus had stood all night,... a candle burned in it, even better than in common air."
A little later Priestley discovered that "dephlogisticated air... is a principal element in the composition of acids, and may be extracted by means of heat from many substances which contain them.... It is likewise produced by the action of light upon green vegetables; and this seems to be the chief means employed to preserve the purity of the atmosphere."
This recognition of the important part played by oxygen in the atmosphere led Priestley to make some experiments upon mice and insects, and finally upon himself, by inhalations of the pure gas. "The feeling in my lungs," he said, "was not sensibly different from that of common air, but I fancied that my breathing felt peculiarly light and easy for some time afterwards. Who can tell but that in time this pure air may become a fashionable article in luxury?... Perhaps we may from these experiments see that though pure dephlogisticated air might be useful as a medicine, it might not be so proper for us in the usual healthy state of the body."
This suggestion as to the possible usefulness of oxygen as a medicine was prophetic. A century later the use of oxygen had become a matter of routine practice with many physicians. Even in Priestley's own time such men as Dr. John Hunter expressed their belief in its efficacy in certain conditions, as we shall see, but its value in medicine was not fully appreciated until several generations later.
Several years after discovering oxygen Priestley thus summarized its properties: "It is this ingredient in the atmospheric air that enables it to support combustion and animal life. By means of it most intense heat may be produced, and in the purest of it animals will live nearly five times as long as in an equal quantity of atmospheric air. In respiration, part of this air, passing the membranes of the lungs, unites with the blood and imparts to it its florid color, while the remainder, uniting with phlogiston exhaled from venous blood, forms mixed air. It is dephlogisticated air combined with water that enables fishes to live in it."(5)
The discovery of oxygen was the last but most important blow to the tottering phlogiston theory, though Priestley himself would not admit it. But before considering the final steps in the overthrow of Stahl's famous theory and the establishment of modern chemistry, we must review the work of another great chemist, Karl Wilhelm Scheele (1742-1786), of Sweden, who discovered oxygen quite independently, although later than Priestley. In the matter of brilliant discoveries in a brief space of time Scheele probably eclipsed all his great contemporaries. He had a veritable genius for interpreting chemical reactions and discovering new substances, in this respect rivalling Priestley himself. Unlike Priestley, however, he planned all his experiments along the lines of definite theories from the beginning, the results obtained being the logical outcome of a predetermined plan.
Scheele was the son of a merchant of Stralsund, Pomerania, which then belonged to Sweden. As a boy in school he showed so little aptitude for the study of languages that he was apprenticed to an apothecary at the age of fourteen. In this work he became at once greatly interested, and, when not attending to his duties in the dispensary, he was busy day and night making experiments or studying books on chemistry. In 1775, still employed as an apothecary, he moved to Stockholm, and soon after he sent to Bergman, the leading chemist of Sweden, his first discovery—that of tartaric acid, which he had isolated from cream of tartar. This was the beginning of his career of discovery, and from that time on until his death he sent forth accounts of new discoveries almost uninterruptedly. Meanwhile he was performing the duties of an ordinary apothecary, and struggling against poverty. His treatise upon Air and Fire appeared in 1777. In this remarkable book he tells of his discovery of oxygen—"empyreal" or "fire-air," as he calls it—which he seems to have made independently and without ever having heard of the previous discovery by Priestley. In this book, also, he shows that air is composed chiefly of oxygen and nitrogen gas.
Early in his experimental career Scheele undertook the solution of the composition of black oxide of manganese, a substance that had long puzzled the chemists. He not only succeeded in this, but incidentally in the course of this series of experiments he discovered oxygen, baryta, and chlorine, the last of far greater importance, at least commercially, than the real object of his search. In speaking of the experiment in which the discovery was made he says:
"When marine (hydrochloric) acid stood over manganese in the cold it acquired a dark reddish-brown color. As manganese does not give any colorless solution without uniting with phlogiston (probably meaning hydrogen), it follows that marine acid can dissolve it without this principle. But such a solution has a blue or red color. The color is here more brown than red, the reason being that the very finest portions of the manganese, which do not sink so easily, swim in the red solution; for without these fine particles the solution is red, and red mixed with black is brown. The manganese has here attached itself so loosely to acidum salis that the water can precipitate it, and this precipitate behaves like ordinary manganese. When, now, the mixture of manganese and spiritus salis was set to digest, there arose an effervescence and smell of aqua regis."(6)
The "effervescence" he refers to was chlorine, which he proceeded to confine in a suitable vessel and examine more fully. He described it as having a "quite characteristically suffocating smell," which was very offensive. He very soon noted the decolorizing or bleaching effects of this now product, finding that it decolorized flowers, vegetables, and many other substances.
Commercially this discovery of chlorine was of enormous importance and the practical application of this new chemical in bleaching cloth soon supplanted the old process of crofting—that is, bleaching by spreading the cloth upon the grass. But although Scheele first pointed out the bleaching quality of his newly discovered gas, it was the French savant, Berthollet, who, acting upon Scheele's discovery that the new gas would decolorize vegetables and flowers, was led to suspect that this property might be turned to account in destroying the color of cloth. In 1785 he read a paper before the Academy of Sciences of Paris, in which he showed that bleaching by chlorine was entirely satisfactory, the color but not the substance of the cloth being affected. He had experimented previously and found that the chlorine gas was soluble in water and could thus be made practically available for bleaching purposes. In 1786 James Watt examined specimens of the bleached cloth made by Berthollet, and upon his return to England first instituted the process of practical bleaching. His process, however, was not entirely satisfactory, and, after undergoing various modifications and improvements, it was finally made thoroughly practicable by Mr. Tennant, who hit upon a compound of chlorine and lime—the chloride of lime—which was a comparatively cheap chemical product, and answered the purpose better even than chlorine itself.
To appreciate how momentous this discovery was to cloth manufacturers, it should be remembered that the old process of bleaching consumed an entire summer for the whitening of a single piece of linen; the new process reduced the period to a few hours. To be sure, lime had been used with fair success previous to Tennant's discovery, but successful and practical bleaching by a solution of chloride of lime was first made possible by him and through Scheele's discovery of chlorine.
Until the time of Scheele the great subject of organic chemistry had remained practically unexplored, but under the touch of his marvellous inventive genius new methods of isolating and studying animal and vegetable products were introduced, and a large number of acids and other organic compounds prepared that had been hitherto unknown. His explanations of chemical phenomena were based on the phlogiston theory, in which, like Priestley, he always, believed. Although in error in this respect, he was, nevertheless, able to make his discoveries with extremely accurate interpretations. A brief epitome of the list of some of his more important discoveries conveys some idea, of his fertility of mind as well as his industry. In 1780 he discovered lactic acid,(7) and showed that it was the substance that caused the acidity of sour milk; and in the same year he discovered mucic acid. Next followed the discovery of tungstic acid, and in 1783 he added to his list of useful discoveries that of glycerine. Then in rapid succession came his announcements of the new vegetable products citric, malic, oxalic, and gallic acids. Scheele not only made the discoveries, but told the world how he had made them—how any chemist might have made them if he chose—for he never considered that he had really discovered any substance until he had made it, decomposed it, and made it again.
His experiments on Prussian blue are most interesting, not only because of the enormous amount of work involved and the skill he displayed in his experiments, but because all the time the chemist was handling, smelling, and even tasting a compound of one of the most deadly poisons, ignorant of the fact that the substance was a dangerous one to handle. His escape from injury seems almost miraculous; for his experiments, which were most elaborate, extended over a considerable period of time, during which he seems to have handled this chemical with impunity.
While only forty years of age and just at the zenith of his fame, Scheele was stricken by a fatal illness, probably induced by his ceaseless labor and exposure. It is gratifying to know, however, that during the last eight or nine years of his life he had been less bound down by pecuniary difficulties than before, as Bergman had obtained for him an annual grant from the Academy. But it was characteristic of the man that, while devoting one-sixth of the amount of this grant to his personal wants, the remaining five-sixths was devoted to the expense of his experiments.
The time was ripe for formulating the correct theory of chemical composition: it needed but the master hand to mould the materials into the proper shape. The discoveries in chemistry during the eighteenth century had been far-reaching and revolutionary in character. A brief review of these discoveries shows how completely they had subverted the old ideas of chemical elements and chemical compounds. Of the four substances earth, air, fire, and water, for many centuries believed to be elementary bodies, not one has stood the test of the eighteenth-century chemists. Earth had long since ceased to be regarded as an element, and water and air had suffered the same fate in this century. And now at last fire itself, the last of the four "elements" and the keystone to the phlogiston arch, was shown to be nothing more than one of the manifestations of the new element, oxygen, and not "phlogiston" or any other intangible substance.
In this epoch of chemical discoveries England had produced such mental giants and pioneers in science as Black, Priestley, and Cavendish; Sweden had given the world Scheele and Bergman, whose work, added to that of their English confreres, had laid the broad base of chemistry as a science; but it was for France to produce a man who gave the final touches to the broad but rough workmanship of its foundation, and establish it as the science of modern chemistry. It was for Antoine Laurent Lavoisier (1743-1794) to gather together, interpret correctly, rename, and classify the wealth of facts that his immediate predecessors and contemporaries had given to the world.
The attitude of the mother-countries towards these illustrious sons is an interesting piece of history. Sweden honored and rewarded Scheele and Bergman for their efforts; England received the intellectuality of Cavendish with less appreciation than the Continent, and a fanatical mob drove Priestley out of the country; while France, by sending Lavoisier to the guillotine, demonstrated how dangerous it was, at that time at least, for an intelligent Frenchman to serve his fellowman and his country well.
