A CENTURY OF CHEMISTRY. 107 



because these external properties are themselves the 

 result of the properties of the real elements forming 

 the isolated elements or the compound. To explain 

 and express the periodic law is to explain and express 

 the cause of the law of multiple proportions, of the 

 difference of the elements, and the variation of their 

 atomicity, and at the same time to understand what 

 mass and gravitation are. In my opinion this is now 

 premature. But just as, without knowing the cause 

 of gravitation, it is possible to make use of the law of 

 gravity, so for the aims of chemistry it is possible to 

 take advantage of the laws discovered by chemistry 

 without being able to explain their causes. The 

 above-mentioned peculiarity of the laws of chemistry 

 respecting definite compounds and the atomic 

 weights leads one to think that the time has not yet 

 come for their full explanation, and I do not think 

 that it will come before the explanation of such pri- 

 mary laws of nature as the law of gravity." * 



The general idea of Mendelejeff's periodic law is 

 that the properties of the elements are periodic func- 

 tions of their atomic iveights, but while this is a 

 simplifying concept it is not in any way an expla- 

 nation. 



The Problem of Chemical Classification. The 

 desire for orderly grouping is one of the mainsprings 

 of scientific work. Even artificial classifications 

 like the grouping of flowers according to the number 

 of their stamens have often justified themselves, 

 though they are apt to outlive their usefulness. It is 

 plain that natural classifications based on deep- 

 seated resemblances must economise thought and 

 make our outlook on the world clearer. Therefore 



* D. Mendelejeff . The FYinc-iples of Chemistry, trans. 

 1897, Vol. II., pp. 20-21, foot-note. 



