Mr. M. M. rattitfoii Muir on Chemical Clat^.si^cation. 93 



general chemical properties with the composition of the com- 

 pound radicle ? It required the genius of a Berzelius to pierce 

 the veil which shrouded the inner constitution of bodies from 

 the vulgar gaze, and to tell authoritatively, by the juxtaposition 

 of what radicles the substance was built up. The upholders 

 of substitution and of the unitary theory endeavoured to cor- 

 relate together general chemical function and atomic structure: 

 and in so doing they made a step in advance ; for they took a 

 wider -s iew of the necessities of a perfect scheme of chemical 

 classification. But general chemical function is a somewhat 

 vague term ; and the use of types as a means of classification, 

 although doubtless a distinct gain to chemical science, ahnost 

 for the time increased the vagueness. I have already pointed 

 out some of the essential weaknesses of a classification by types; 

 these weaknesses Avere illustrated in the case of chemistry. 

 From the vagueness of a typical classification we may be said 

 to have now emerged into the greater definiteness of a system 

 largely based upon the valency of the elementary atoms. But 

 does this system give us a means for definitely settling the 

 characteristics of each class ? Does it enable us to mark off class 

 from class ? Can we by its aid correlate the leading charac- 

 teristics of each class with the valency of the elementary 

 atoms ? And, lastly, is there a clear connexion between valency 

 and those chemico-physical properties of elements and com- 

 pounds which modern chemistry has brought, and is every day 

 bringing, into so great a prominence ? I shall endeavour 

 shortly to answer some of these questions. 



12. But we cannot fairly regard the idea of valency as a 

 basis for classification apart from those other ideas to which it 

 has given rise, notably the idea of atom-linking or molecular 

 structure. Let us therefore shortly review the more important 

 ideas connected with this term valency*. 



And at the outset it is well to bear in mind that the valency 

 of an element is a pure number ; it is the number obtained by 

 dividing the atomic weight by the equivalent of the element. 

 The determination of the valency of an element therefore presup- 

 poses the determination of the atomic weight and of the equiva- 

 lent of that element. Those weights of different elements are 

 said to be equivalent which are capable of binding to themselves 

 equal weights of a third substance, immediately, and without the 

 intervention of another body. Hydrogen is the unit of equiva- 

 lency. We know of certain elements which combine imme- 

 diately with hydrogen in the proportion of one atom to one 



* In preparing this part of my paper I have made fi-ee use of that great 

 work on theoretical chemistry, Die inodernen Theorien der Chemie, by 

 Lothar Meyer. 



