THE CAUSATION OF DISEASE. 9 



chemical action. All chemical action is governed by " material 

 conditions/' and the material conditions of which chemical 

 action is the outcome are — (i) The presence of certain kinds 

 of atoms. (2) The arrangements of these atoms — i.e., their 

 relation one to another. (3) The kind of motion by which 

 these atoms, and -the molecules formed of them, are agitated — 

 namely, heat, light, electricity and such like modes of .motion. 

 In seeking for the " cause " of any particular chemical change, 

 we can do no more than discover the material conditions as just 

 set forth. These, as Bain would say, " are the final terms of 

 the explanation." Viewing the matter thus, we see how little 

 is the difference between astronomy and chemistry. We must 

 not, in our blindness, despise the tiny atom, for, in the eye of the 

 natural philosopher, it is as important as any great world rolling 

 through space, and, indeed, the analogies between the two are 

 very great : both world and atom are indestructible and eternal ; 

 each is governed by fixed and unchangeable law ; and, if an 

 atom is small by the side of the world, so is a world small 

 by the side of the universe. A group of worlds, arranged 

 in a planetary system, is akin to a group of atoms forming a 

 molecular system ; the atom, like the world, is the seat of 

 movements both about its own axis and about a point outside 

 itself; finally, both the planetary and the molecular systems 

 have a definite structure, which may be defined as the mutual 

 positions which the constituent parts, whether worlds or atoms, 

 bear to one another. 



That the structure, or mutual position of the several atoms 

 in the molecule determines very largely the nature of that 

 molecule, is well shown in the case of the so-called isomeric 

 bodies — that is to say, unlike bodies whose molecules contain 

 the same number of the same kind of atoms. The primary, 

 secondary, and tertiary alcohols afford a familiar example. 

 These are isomeric, but each differs from the other in its 

 chemical properties, and chemists have actually attempted to 

 determine the molecular structure in each case. Thus : 



