458 



Mr. E. Wilson. 



the simple monatomic molecule of any element must have a determinate 

 and invariable volume if the element conld be reduced to the monatomic 

 condition, and this volume may be defined as its atomic volume. But 

 an element does not carry its atomic volume unchanged into its com- 

 pounds in the same way that it carries its atomic weight ; indeed, it is 

 not probable that an atom of a compound molecule has any volume at 

 all (except, of course, in so far as the mere matter of the atom may 

 have a volume) in the sense of occupying exclusively of other atoms 

 a discrete portion of space. The whole volume of the molecule is 

 shared by its constituent atoms in common, and no separate portion 

 of its space can be assigned exclusively to any one particular atom ; 

 nevertheless, each atom of the molecule must play its part in the 

 formation of the common volume, and therefore a certain propor- 

 tion of that volume may be attributed to each atom in it. 



The invariable and all- important atomic volumes defined in the 

 preceding paragraph cannot be the subject of direct experimental 

 investigation, except perhaps in the case of the few elements, such as 

 mercury, cadmium, and zinc, which are known to be monatomic in the 

 gaseous state at terrestrial temperatures, though all the elements may 

 be conceived as capable of thus existing under suitable conditions. 

 The atomic volumes must in the case of each element be deduced from 

 a comparison of the specific gravities of the various compounds in 

 which that element figures as a constituent. The first point to be 

 aimed at is to discover, by means of comparisons, the values of these 

 atomic volumes, because they of necessity form the only sound and 

 rational basis of all speculations on the volumes of compound molecules. 

 There exists no other firm ground or secure starting point ; atomic 

 volumes are to molecular volumes what atomic weights are to mole- 

 cular weights. 



When two or more atoms combine to form a chemical compound, a 

 very intimate union of some sort takes place between the atoms, of the 

 real nature of which we are, in the present state of science, pro- 

 foundly ignorant ; but at any rate, a new molecule is formed with a 

 new volume, and the question arises as to what relations subsist 

 between this new volume and the atomic volumes of the components 

 of the molecule. This is the problem which it is sought to solve, and 

 the answer, perhaps, may best be given by the enunciation of the two 

 following propositions : — 



(1.) When any number of similar atoms combine, the volume of 

 the resulting molecule is equal to that of the uncombined atom. 



(2.) When dissimilar atoms combine, the volume to be attributed to 

 each atom is some submultiple or simple aliquot part of its atomic 

 volume, and the resulting molecular volume is the sum of these. 



The somewhat speculative character of the above views will not 

 escape the notice of any one, but before entering upon an explanation 



