238 THE NUMERICAL RELATION 



would not be more absurd, than it is for a chemist to separate two essentially allied 

 elements, because one has a metallic lustre and the other has not. Yet it is surpris- 

 ing to see how persistently this classification is retained in every elementary work on 

 the science ; and if it is sometimes so far modified as to transfer elements analogous 

 to Selenium and Ai'senic to the class of metalloids, this is only acknowledging the 

 worthlessness of the principle, without being willing to abandon it. If there were 

 any fundamental property common to all the elements, the law of whose variation was 

 known, this might serve as the basis of a correct classification. Chemistry, however, 

 does not as yet present us with such a property, and we must, therefore, here, as in 

 other sciences, base our classification on general analogies. The most fundamental of 

 all chemical properties is, undoubtedly, crystalline form, but a classification of the 

 elements based solely on the principles of isomorphism is defective in the same way 

 as it is in mineralogy. It brings together, undoubtedly, allied elements, but it also 

 groups with them those which resemble each other only in their crystalline form. 

 The mode of combining seems to be also a fundamental property ; but, like crystalline 

 form, it would bring together in some instances elements difi'ering very widely in their 

 chemical properties. A classification of the elements which shall exhibit their nat- 

 ural affinities, must obviously pay regard to both of these properties. It must at the 

 same time seek to group together isomorphous elements, and those which form anal- 

 ogous compounds. Moreover, in such a classification, other less fundamental prop- 

 erties must not be disregarded. There are many properties both physical and chem- 

 ical, which, although they cannot be exactly measured, and are oftentimes difficult 

 to define, (such properties as those by which a chemist recognizes a familiar substance, 

 or a mineralogist a familiar mineral,) and which on account of their indefinite char- 

 acter cannot be used as a basis of classification, may, nevertheless, render important 

 aid in tracing out analogies. Judging from such properties as these, chemists are 

 generally agreed in grouping together Carbon, Boron, and Silicon, although they can- 

 not be ]3roved to be isomorphous, and are not generally thought to form similar 

 compounds. 



It is, however, much easier to point out what a classification should be, than to 

 make one which shall fulfil the required conditions. Indeed, as has been already 

 said, past experience would seem to show that a perfect scientific classification of the 

 elements is hardly possible in the present state of Chemistry. At best, the task is 

 attended with great difficulties, and it cannot be expected that these should be sur- 

 mounted at once. The classification which is offered in this memoir will, undoubtedly, 

 be found to contain many defects. If, however, it is but one step in advance of those 



