THE LAWS OF MOTION 279 



be conceived as constituting the prime -atom. These 

 prime-atoms, the protyle of Crookes, are to be taken as 

 symbols of the ultimate basis of material groups of sense- 

 impressions, or, in ordinary language, of gross or sensible 

 " matter." * Prime-atoms in themselves, or, what is more 

 likely, in groups, form the atom of the chemist, the 

 conceptual substratum of the so-called simple elements 

 such as hydrogen, oxygen, iron, carbon, etc., by aid of 

 which the chemist classifies all the known heavy matter of 

 the physical universe. If the prime-atom of the physicist 

 is really the atom of the chemist, then the prime-atom 

 must be conceived as having variations either in its 

 structure or in its type of motion corresponding to the 

 different chemical elements. There are certain perceptual 

 facts, however, which suggest that we should describe 

 phenomena best by conceiving the atom of the simple 

 chemical element to be constructed from groups of prime- 

 atoms, the disassociation of which corresponds to no definite 

 perceptual results which the chemist has hitherto succeeded 

 in attaining. Out of the atoms of the simple elements the 

 chemist constructs compounds ; that is, by combining 

 conceptually these atoms in certain groupings he forms 

 the molecule of the compound. Thus two atoms of hydrogen 

 and one of oxygen are united to form the molecule of 

 water. Any portion of the compound substance itself is 

 conceived as composed of an immense number of molecules. 

 In order to describe the sense-impressions which we 

 physically associate with a " piece of a given substance " 

 we are bound to postulate that the smallest physical 

 element of it is to be considered as containing millions of 

 molecules.^ 



^ The reasons for this statement are chiefly drawn from the Kinetic 

 Theory of Gases. Clerk-Maxwell in his article "Atom" {Encyclopcedia 

 Briiannica) considers that the minimum visibile of the present day may be 

 conceived as containing sixty to one hundred million atoms of oxygen or 

 nitrogen. He proceeds to draw from this result conclusions, which I think 

 quite unwarranted, as to our power of describing by aid of molecular structure 

 the physiological facts of heredity. He remarks that : " Since the molecules 

 of organised substances contain on an average fifty of the more elementary 

 atoms, we may assume that the smallest particle visible under the microscope 

 contains about two million molecules of organic matter. At least half of 

 every living organism consists of water, so that the smallest living being 



