CHEMISTRY 85 



formations — something entirely new in military tactics but 

 the rule in chemical reactions. The chemical analog}^ here is 

 that the molecules, the component units of substances, are 

 structures and that when substances are destroyed the ulti- 

 mate structural units, the chemical atoms, are not destroyed. 



Of these atoms, which are the chemical elements, between 

 seventy and one hundred different kinds are recognized by 

 chemists, each kind being distinct from every other kind, pos- 

 sessing its own mass and volume, the mass and volume being 

 the same for all atoms of one kind, all kinds being assumed 

 indestrucible and indivisible. Their existence, indestructibility 

 and indivisibility are inferred from the behavior of substances 

 taking part in a chemical reaction.* 



To those chemists who believed in the existence of the 

 atom (not all chemists believed in its existence) the atom was 

 of determinable volume (size) and mass (weight), and struc- 

 tureless. It was a unit of matter — matter being that which 

 occupies space and all the space it seems to occupy, matter 

 bearing a relation to the molecule similar to that which the 

 popped grains of corn do to a ball of popped corn. These 

 molecules form substance — the substance possessing a granu- 

 lar structure or perhaps a "bird-cage kind of structure," the 

 granules, the molecules composed of indestructible, indivisible, 

 homogeneous masses, the atoms, matter. 



The size of these atoms and their number have been ap- 

 proximately determined in various ways. Among the first 

 of these attempts was that of Lord Kelvin (then Sir William 

 Thomson) who calculated from various data that in any or- 

 dinary liquid, transparent solid or seemingly opaque solid the 

 molecules were apart, and, "To form some conception of the 

 degree of coarse-grainedness indicated — imagine a rain drop. 



♦There is a very interesting paper by Dr. J. "W. Nicholson on 

 "A Structural Theory of the Chemical Elements" in the Philosophi- 

 cal Magazine, Vol. XXII, p. 864, 1911, "which derives them as 

 compounds to a certain extent, of primary forms of matter." 



