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.
If Lavoisier's memoirs are examined closely, it is seen that at the very beginning of his chemical inquiries he assumed the accuracy, and the universal application, of the generalisation "nothing is created, either in the operations of art or in those of nature." Naturalists had been feeling their way for centuries towards such a generalisation as this; it had been in the air for many generations; sometimes it was almost realised by this or that investigator, then it escaped for long periods.

Lavoisier seems to have realised, by what we call intuition, that however great and astonishing may be the changes in the properties of the substances which mutually react, there is no change in the total quantity of material.
Not only did Lavoisier realise and act on this principle, he also measured quantities of substances by the one practical method, namely, by weighing; and by doing this he showed chemists the only road along which they could advance towards a genuine knowledge of material changes.
The generalisation expressed by Lavoisier in the words I have quoted is now known as the _law of the conservation of mass_; it is generally stated in some such form as this:--the sum of the masses of all the homogeneous substances which take part in a chemical (or physical) change does not itself change.

The science of chemistry rests on this law; every quantitative analysis assumes the accuracy, and is a proof of the validity, of it.[11] [11] I have considered the law of the conservation of mass in some detail in Chapter IV.

of _The Story of the Chemical Elements_.
By accepting the accuracy of this generalisation, and using it in every experiment, Lavoisier was able to form a clear mental picture of a chemical change as the separation and combination of homogeneous substances; for, by using the balance, he was able to follow each substance through the maze of changes, to determine when it united with other substances, and when it separated into substances simpler than itself..