Part 6 (2/2)
The explanation given by Lavoisier of combustion was to a great extent based on a conception of element and compound very different from that of the older chemists. In the ”Sceptical Chymist” (1661) Boyle had argued strongly against the doctrine of the four ”elementary principles,” earth, air, fire and water, as held by the ”vulgar chymists.” The existence of these principles, or some of them, in every compound substance was firmly held by most chemists in Boyle's time. They argued thus: when a piece of green wood b.u.ms, the existence in the wood of the principle of fire is made evident by the flame, of the principle of air by the smoke which ascends, of that of water by the hissing and boiling sound, and of the principle of earth by the ashes which remain when the burning is finished.[6]
Boyle combated the inference that because a flame is visible round the burning wood, and a light air or smoke ascends from it, _therefore_ these principles were contained in the wood before combustion began. He tried to prove by experiments that one substance may be obtained from another in which the first substance did not already exist; thus, he heated water for a year in a closed gla.s.s vessel, and obtained solid particles heavier than, and as he supposed formed from, the water. We have already learned the true interpretation of this experiment from the work of Lavoisier. Boyle grew various vegetables in water only, and thought that he had thus changed water into solid vegetable matter. He tells travellers' tales of the growth of pieces of iron and other metals in the earth or while kept in underground cellars.
We now know how erroneous in most points this reasoning was, but we must admit that Boyle established one point most satisfactorily, viz. that because earth, or air, or fire, or water is obtained by heating or otherwise decomposing a substance, it does not necessarily follow that the earth, or air, or fire, or water existed as such in the original substance.
He overthrew the doctrine of elementary principles held by the ”vulgar chymists.” Defining elements as ”certain primitive and simple bodies which, not being made of any other bodies, or of one another, are the ingredients of which all those called perfectly mixt bodies are immediately compounded, and into which they are ultimately resolved,” Boyle admitted the _possible_ existence, but thought that the facts known at his time did not warrant the a.s.sertion of the _certain_ existence, of such ”elements.” The work of Hooke and Mayow on combustion tended to strengthen this definition of ”element”
given by Boyle.
Black, as we have seen, clearly proved that certain chemical substances were possessed of definite and unvarying composition and properties; and Lavoisier, indirectly by his explanation of combustion, and directly in his ”Treatise on Chemistry”, laid down the definition of ”element” which is now universally adopted.
An element is a substance from which no simpler forms of matter--that is, no forms of matter each weighing less than the original substance--have _as yet_ been obtained.
In the decade 1774-1784 chemical science was thus established on a sure foundation by Lavoisier. Like most great builders, whether of physical or mental structures, he used the materials gathered by those who came before him, but the merit of arranging these materials into a well-laid foundation, on which the future building might firmly rest, is due to him alone.
The value of Lavoisier's work now began to be recognized by his fellow-chemists in France. In 1785 Berthollet, one of the most rising of the younger French chemists, declared himself a convert to the views of Lavoisier on combustion. Fourcroy, another member of the Academy, soon followed the example of Berthollet. Fourcroy, knowing the weakness of his countrymen, saw that if the new views could be made to appear as especially the views of Frenchmen, the victory would be won; he therefore gave to the theory of Lavoisier the name ”_La chimie Francaise_”. Although this name was obviously unfair to Lavoisier, it nevertheless caused the antiphlogistic theory to be identified with the French chemists, and succeeded in impressing the French public generally with the idea that to hold to the old theory was to be a traitor to the glory of one's country.
M. de Morveau, who held a prominent place both in politics and science, was invited to Paris, and before long was persuaded to embrace the new theory.
This conversion--for ”the whole matter was managed as if it had been a political intrigue rather than a philosophical inquiry”--was of great importance to Lavoisier and his friends. M. de Morveau was editor of the chemical part of the ”Encyclopedie Methodique;” in that part of this work which had appeared before 1784 De Morveau had skilfully opposed the opinions of Lavoisier, but in the second part of the work he introduced an advertis.e.m.e.nt announcing the change in his opinions on the subject of combustion, and giving his reasons for this change.
The importance of having a definite language in every science is apparent at each step of advance. Lavoisier found great difficulty in making his opinions clear because he was obliged to use a language which had been introduced by the phlogistic chemists, and which bore the impress of that theory on most of its terms. About the years 1785-1787, Lavoisier, Berthollet, Fourcroy and De Morveau drew up a new system of chemical nomenclature. The fundamental principles of that system have remained as those of every nomenclature since proposed. They are briefly these:--
An element is a substance from which no form of matter simpler than itself has as yet been obtained.
Every substance is to be regarded as an element until it is proved to be otherwise.
The name of every compound is to tell of what elements the substance is composed, and it is to express as far as possible the relative amounts of the elements which go to form the compound.
Thus the compounds of oxygen with any other element were called oxides, _e.g._ iron oxide, mercury oxide, tin oxide, etc. When two oxides of iron came to be known, one containing more oxygen relatively to the amount of iron present than the other, that with the greater quant.i.ty of oxygen was called iron peroxide, and that with the smaller quant.i.ty iron protoxide.
We now generally prefer to use the name of the element other than oxygen in adjectival form, and to indicate the relatively smaller or greater quant.i.ty of oxygen present by modifications in the termination of this adjective.
Thus iron protoxide is now generally known as ferr_ous_ oxide, and iron peroxide as ferr_ic_ oxide. But the principles laid down by the four French chemists in 1785-1787 remain as the groundwork of our present system of nomenclature.
The antiphlogistic theory was soon adopted by all French chemists of note.
We have already seen that Black, with his usual candour and openness to conviction, adopted and taught this theory, and we are a.s.sured by Dr.
Thomas Thomson that when he attended Black's cla.s.ses, nine years after the publication of the French system of nomenclature, that system was in general use among the chemical students of the university. The older theory was naturally upheld by the countrymen of the distinguished Stahl after it had been given up in France. In the year 1792 Klaproth, who was then Professor of Chemistry in Berlin, proposed to the Berlin Academy of Sciences to repeat the more important experiments on which the Lavoisierian theory rested, before the Academy. His offer was accepted, and from that time most of the Berlin chemists declared themselves in favour of the new theory.
By the close of last century the teaching of Lavoisier regarding combustion found almost universal a.s.sent among chemists. But this teaching carried with it, as necessary parts, the fundamental distinction between element and compound; the denial of the existence of ”principles” or ”essences;”
the recognition of the study of actually occurring reactions between substances as the basis on which all true chemical knowledge was to be built; and the full acknowledgment of the fact that matter is neither created nor destroyed, but only changed as to its form, in any chemical reaction.
Of Lavoisier's other work I can only mention the paper on ”Specific Heats”
contributed by Laplace and Lavoisier to the Memoirs of the Academy for 1780. In this paper is described the ice calorimeter, whereby the amount of heat given out by a substance in cooling from one definite temperature to another is determined, by measuring the amount of ice converted into water by the heated substance in cooling through the stated interval of temperature. The specific heats of various substances, _e.g._ iron, gla.s.s, mercury, quicklime, etc., were determined by the help of this instrument.
As we read the record of work done by Lavoisier during the years between 1774 and 1794--work which must have involved a great amount of concentrated thought as well as the expenditure of much time--we find it hard to realize that the most tremendous political and social revolution which the modern world has seen was raging around him during this time.
In the earlier days of the French Revolution, and in the time immediately preceding that movement, many minds had been stirred to see the importance of the study of Nature; but it was impossible that natural science should continue to flourish when the tyrant Robespierre had begun the Reign of Terror.
The roll of those who perished during this time contains no more ill.u.s.trious name than that of Antoine Laurent Lavoisier. In the year 1794 Lavoisier, who had for some time acted as a _fermier-general_ under the Government, was accused of mixing with the tobacco ”water and other ingredients hurtful to the health of the citizens.” On this pretext he and some of his colleagues were condemned to death. For some days Lavoisier found a hiding-place among his friends, but hearing that his colleagues had been arrested, he delivered himself up to the authorities, only asking that the death sentence should not be executed until he had completed the research in which he was engaged; ”not” that he was ”unwilling to part with life,” but because he thought the results would be ”for the good of humanity.”
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