him; he was still inclined to say, “this must be the way nature works, she must begin with certain substances which are absolutely simple.”  Lavoisier had thrown off all the trammels which hindered the alchemists from making rigorous experimental investigations.  If one may judge from his writings, he had not struggled to free himself from these trammels, he had not slowly emerged from the quagmires of alchemy, and painfully gained firmer ground; the extraordinary clearness and directness of his mental vision had led him straight to the very heart of the problems of chemistry, and enabled him not only calmly to ignore all the machinery of Elements, Principles, Essences, and the like, which the alchemists had constructed so laboriously, but also to construct, in place of that mechanism which hindered inquiry, genuine scientific hypotheses which directed inquiry, and were themselves altered by the results of the experiments they had suggested.

Lavoisier made these great advances by applying himself to the minute and exhaustive examination of a few cases of chemical change, and endeavouring to account for everything which took part in the processes he studied, by weighing or measuring each distinct substance which was present when the change began, and each which was present when the change was finished.  He did not make haphazard experiments; he had a method, a system; he used hypotheses, and he used them rightly.  “Systems in physics,” Lavoisier writes, “are but the proper instruments for helping the feebleness of our senses.  Properly speaking, they are methods of approximation which put us on the track of solving problems; they are the hypotheses which, successively modified, corrected, and changed, by experience, ought to conduct us, some day, by the method of exclusions and eliminations, to the knowledge of the true laws of nature.”

In a memoir wherein he is considering the production of carbonic acid and alcohol by the fermentation of fruit-juice, Lavoisier says, “It is evident that we must know the nature and composition of the substances which can be fermented and the products of fermentation; for nothing is created, either in the operations of art or in those of nature; and it may be laid down that the quantity of material present at the beginning of every operation is the same as the quantity present at the end, that the quality and quantity of the principles[9] are the same, and that nothing happens save certain changes, certain modifications.  On this principle is based the whole art of experimenting in chemistry; in all chemical experiments we must suppose that there is a true equality between the principles[10] of the substances which are examined and those which are obtained from them by analysis.”

   [9, 10] Lavoisier uses the word principle, here and
   elsewhere, to mean a definite homogeneous substance; he uses it
   as synonymous with the more modern terms element and compound.