Heroes of Science: Chemists

by M. M. Pattison Muir

But before considering the work of Lavoisier, I should like to point out that many of the physical characters of common air had been clearly established in the later years of the seventeenth century by the Honourable Robert Boyle. In the "Sceptical Chymist," published in 1661, Mr. Boyle had established the fact that air is a material substance possessed of weight, that this air presses on the surface of all things, and that by removing part of the air in an enclosed space the pressure within that space is diminished. He had demonstrated that the boiling point of water is dependent on the pressure of the air on the surface of the water. Having boiled some water "a pretty while, that by the heat it might be freed from the latitant air," he placed the vessel containing the hot water within the receiver of an arrangement which he had invented for sucking air out of an enclosed space; as soon as he began to suck out air from this receiver, the water boiled "as if it had stood over a very quick fire.... Once, when the air had been drawn out, the liquor did, upon a single exsuction, boil so long with prodigiously vast bubbles, that the effervescence lasted almost as long as was requisite for the rehearsing of a Pater noster." Boyle had gone further than the qualitative fact that the volume of an enclosed quantity of air alters with changes in the pressure to which that air is subjected; he had shown by simple and accurate experiments that "the volume varies inversely as the pressure." He had established the generalization of so much importance in physical science now known as Boyle's law.

The work of the Honourable Henry Cavendish will be considered in some detail in the book on "The Physicists" belonging to this series, but I must here briefly allude to the results of his experiments on air published in the Philosophical Transactions for 1784 and 1785.

Cavendish held the ordinary view that when a metal burns in air, the air is thereby phlogisticated; but why is it, he asked, that the volume of air is decreased by this process? It was very generally said that fixed air was produced during the calcination of metals, and was absorbed by the calx. But Cavendish instituted a series of experiments which proved that no fixed air could be obtained from metallic calces. In 1766 inflammable air (hydrogen) was discovered by Cavendish; he now proved that when this air is exploded with dephlogisticated air (oxygen), water is produced. He showed that when these two airs are mixed in about the proportion of two volumes of hydrogen to one volume of oxygen, the greater part, if not the whole of the airs is condensed into water by the action of the electric spark. He then proceeded to prove by experiments that when common air is exploded with inflammable air water is likewise produced, and phlogisticated air (i.e. nitrogen) remains.

Priestley and Cavendish had thus distinctly established the existence of three kinds of air, viz. dephlogisticated air, phlogisticated air, and inflammable air. Cavendish had shown that when the last named is exploded with common air water is produced (which is composed of dephlogisticated and inflammable airs), and phlogisticated air remains. Common air had thus been proved to consist of these two—phlogisticated and dephlogisticated airs (nitrogen and oxygen). Applying these results to the phenomenon of the calcination of metals, Cavendish gave reasons for thinking that the metals act towards common air in a manner analogous to that in which inflammable air acts—that they withdraw dephlogisticated and leave phlogisticated air; but, as he was a supporter of the phlogistic theory, he rather preferred to say that the burning metals withdraw dephlogisticated air and phlogisticate that which remains; in other words, while admitting that a metal in the process of burning gains dephlogisticated air, he still thought that the metal also loses something; viz. phlogiston.

That Cavendish in 1783-84 had proved air to consist of two distinct gases, and water to be produced by the union of two gases, must be remembered as we proceed with the story of the discoveries of Lavoisier.

Antoine Laurent Lavoisier, born in Paris in 1743, was the son of a wealthy merchant, who, judging from his friendship with many of the men of science of that day, was probably of a scientific bent of mind, and who certainly showed that he was a man of sense by giving his son the best education which he could obtain. After studying in the Mazarin College, Lavoisier entered on a course of training in physical, astronomical, botanical and chemical science. The effects of this training in the accurate methods of physics are apparent in the chemical researches of Lavoisier.

At the age of twenty-one Lavoisier wrote a memoir which gained the prize offered by the French Government for the best and most economical method of lighting the streets of a large city. While making experiments, the results of which were detailed in this paper, Lavoisier lived for six weeks in rooms lighted only by artificial light, in order that his eyesight might become accustomed to small differences in the intensities of light from various sources. When he was twenty-five years old Lavoisier was elected a member of the Academy of Sciences. During the next six years (1768-1774) he published various papers, some on chemical, some on geological, and some on mathematical subjects. Indeed at this time, although an ardent cultivator of natural science, he appears to have been undecided as to which branch of science he should devote his strength.

The accuracy and thoroughness of Lavoisier's work, and the acuteness of his reasoning powers, are admirably illustrated in two papers, published in the Memoirs of the Academy for 1770, on the alleged conversion of water into earth.

When water is boiled for a long time in a glass vessel a considerable quantity of white siliceous earth is found in the vessel. This apparent conversion or transmutation of water into earthy matter was quite in keeping with the doctrines which had been handed down from the times of the alchemists; the experiment was generally regarded as conclusively proving the possibility of changing water into earth. Lavoisier found that after heating water for a hundred and one days in a closed and weighed glass vessel, there was no change in the total weight of the vessel and its contents; when he poured out the water and evaporated it to dryness, he obtained 20.4 grains of solid earthy matter; but he also found, what had been before overlooked, that the glass vessel had lost weight. The actual loss amounted to 17.4 grains. The difference between this and the weight of the earthy matter in the water, viz. three grains, was set down (and as we now know justly set down) by Lavoisier to errors of experiment. Lavoisier therefore concluded that water, when boiled, is not changed into earth, but that a portion of the earthy matter of which glass is composed is dissolved by the water. This conclusion was afterwards confirmed by the Swedish chemist Scheele, who proved that the composition of the earthy matter found in the water is identical with that of some of the constituents of glass.

By this experiment Lavoisier proved the old alchemical notion of transmutation to be erroneous; he showed that water is not transmuted into earth, but that each of these substances is possessed of definite properties which belong to it and to it only. He established the all-important generalization—which subsequent research has more amply confirmed, until it is to-day accepted as the very foundation of every branch of physical science—that in no process of change is there any alteration in the total mass of matter taking part in that change. The glass vessel in which Lavoisier boiled water for so many days lost weight; but the matter lost by the glass was found dissolved in the water.