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. CHAPTER XIII. THE CHEMICAL ELEMENTS CONTRASTED WITH THE ALCHEMICAL PRINCIPLES. It was known to many observers in the later years of the 17th century that the product of the calcination of a metal weighs more than the metal; but it was still possible, at that time, to assert that this fact is of no importance to one who is seeking to give an accurate description of the process of calcination. Weight, which measures mass or quantity of substance, was thought of, in these days, as a property like colour, taste, or smell, a property which was sometimes decreased, and sometimes increased, by adding one substance to another. Students of natural occurrences were, however, feeling their way towards the recognition of some property of substances which did not change in the haphazard way wherein most properties seemed to alter. Lavoisier reached this property at one bound. By his experimental investigations, he taught that, however greatly the properties of one substance may be masked, or altered, by adding another substance to it, yet the property we call mass, and measure by weight, is not affected by these changes; for Lavoisier showed, that the mass of the product of the union of two substances is always exactly the sum of the masses of these two substances, and the sum of the masses of the substances whereinto one substance is divided is always exactly equal to that mass of the substance which is divided.