Theory of Molecular Volumes. 347 



atoms in each group arrange themselves according to their 

 fundamental valencies, or increase continuously to the first 

 group. 



The monovalent atoms like F, CI, Br arrange themselves 

 on the lowest curve. 



The divalent atoms like 0, S, Cr arrange themselves on 

 the next higher. 



The trivalent atoms like N, P, V on the third ; and the 

 tetravalent atoms like C, Si, Ti on the fourth. 



Moreover, the curves are similar. 



This shows that the volume is a function of the atomic 

 weight on the one hand and the fundamental valency on the 

 other, or at least increase continuously in each period. 



The points to notice are the increase with atomic weight 

 in the different groups, and the decrease with the atomic 

 weights in the different periods. 



Moreover, the volume is largely independent of the manner 

 of linking and of changes of valency (vide prox.). 



The regularity is best shown in the atoms of the short 

 periods and the even series of the long periods. 



The Odd series of the long periods show some irregularity, 

 although not of opposition to the above principle. Some 

 allowance must be made for differences in environment. 

 If we could obtain the volumes of a number of simple com- 

 pounds of the elements like the chlorides, we might get a 

 better idea of how the atomic volumes vary in this region. 



Consider the elements in the Odd series of the second long 

 period. 



A more extended examination of the Halogen compounds 

 than Thorpe has made and a systematic study of the organ o- 

 metallic derivatives would certainly lead to results of the 

 highest interest. 



The Additive Factor in Molecidar Volumes. 



That an additive factor in molecular volumes exists is 

 undoubted, although an uncompromising Additive Theory 

 such as was at first advocated by Kopp is untenable. The 

 so-called constants which have from time to time been 

 obtained can only be regarded as average numbers. They 

 nevertheless possess a certain validity provided that we are 

 careful to choose compounds within certain limits of com- 

 plexity which are structurally similar. 



The following simple substances (p. 348) illustrate the 

 additive principle. 



The same numbers are found to apply to organic and 

 inorganic compounds indiscriminately. 



