Page:The American Cyclopædia (1879) Volume IV.djvu/371

 CHEMISTRY 363 tions of numberless phenomena which had previously been known as mere isolated facts. It was the most rational theory of combustion, and of many analogous processes now referred to oxidation and reduction, which had been pro- posed. In a comparatively short time it led to a multitude of discoveries. By regarding bodies as composed of undecomposable elements, it established the view which has ever since been admitted. Indeed, the definition of chemistry given by Stahl, the art of decomposing com- pound bodies and of reproducing them, is en- tirely in accordance with the spirit of the pres- ent day. Far from lamenting the phlogiston theory as an error, it should be regarded as a necessary basis for the views of the present epoch. In order to obtain a correct knowledge of substances, it was essential that their prop- erties and relations to one another should be first investigated qualitatively. In the accom- plishment of this work lies the merit of the phlogiston period. The quantitative method of investigation which followed, and which is characteristic of the present epoch, was but a natural succession. In no instance, however, in the history of chemistry has the transition from one system to another been so abrupt as from phlogiston to anti-phlogiston. Karely has the introduction of an entirely new doctrine been so completely dependent upon one indi- vidual as was that of the present system upon Lavoisier (1743-'94). He first caused the im- portance of the quantitative method of research to be recognized, having by its aid perceived that the increase of weight acquired by a given weight of metal, when oxidized, must disprove the whole theory of phlogiston. The mere fact of this increase of weight had long been known ; even Stahl was familiar with it. He regarded it, however, as entirely unessential and depen- dent on accidental circumstances. His followers were of the same opinion. One or two chemists had indeed called attention to the fact that this increase of weight was caused by an absorption of gas, but they had not themselves perceived the true importance of their observation, and their explanations had remained unnoticed. When at length the more careful investigation of Lavoisier had clearly proved the fact that the oxidation of metals is accompanied by in- crease of weight, the most absurd propositions were resorted to by the supporters of phlogis- ton. Among them it is only necessary to in- stance the hypothesis that phlogiston possessed absolute levity, being a substance endowed with a peculiar repulsive force, tending to re- move it from the earth instead of gravitating toward it like other substances ; whence a body losing phlogiston must become heavier. Lavoi- sier not only exposed the error of supposing that the heavier metallic oxide could be an in- gredient of a metal possessing less weight, but also framed a new theory in explanation of the facts, viz. : that in calcination, as in combustion generally, one of the ingredients of the atmo- sphere unites with the combustible in such proportion that the products of combustion weigh exactly as much as the sum of the weight of the substance consumed plus the weight of the matter absorbed from the air. Still more important was his proof that wher- ever an increase of weight occurs combination must have taken place ; that the weight of the product of such combination is equal to the sum of the weights of its ingredients, while diminution of weight is invariably owing to separation of ponderable matter. As an in- vestigator Lavoisier stands preeminent. His precision of observation, ingenuity in devising apparatus, and his patience, are only equalled by the clearness of his conclusions and his masterly description of facts. His original ex- periment, showing that the increase of weight which a metal acquires when calcined is caused by its combination with a gas, deserves to be mentioned. A known quantity of metallic tin having been placed in a retort, the latter was sealed up and the whole weighed. Heat was then applied until the tin, having melted, was converted to a greater or less extent into a calx. On again observing the weight of the apparatus, it was found to have remained un- changed. But on opening the retort, air rushed in, and on again weighing an increase of weight was found to have occurred. This excess of weight showed how much air had entered the retort to replace what had been ab- sorbed, and it was found that the weight of the tin had been increased by the same quan- tity. After the discovery of oxygen by Priest- ley, Lavoisier refined this experiment by using mercury in place of tin. Continuing his ob- servations on oxygen, he found that carbonic acid is a compound of it and carbon, and ex- plained the combustion of organic substances. He also studied the composition of sulphuric and other acids, seeking to multiply proof of his view that oxygen is the universal acidi- fying principle. Cavendish's discovery that hydrogen when burned forms water, afforded Lavoisier a clue for the explanation of the so- lution of metals in acids, which had previously been the weak point of his theory. He rec- ognized at once that decomposition of water must take place, its hydrogen being set free, while the oxygen unites with the metal. He also prepared water synthetically, and analyzed it by passing its vapor over red-hot iron, with which the oxygen united while hydrogen was set free, thus fixing its composition beyond the possibility of doubt. Lavoisier distinguished among the chemical elements those which ought to be regarded as simple, their further decomposition being improbable, as light, heat, oxygen, hydrogen, and nitrogen. Others h< considered not so much simple as undecom posed, their ingredients not yet being known. In this class he placed the alkalies, earths, and metals. The radicals of the acids he supposed to be simple; of these, sulphur, carbon, and phosphorus were known, while the radicals of boracic, muriatic, and hydrofluoric acids