-Manchester Memoirs^ Vol Hi. (1908), No. 10. 15 



Even in chemical philosophy it has at times been a 

 matter of concern that, for example, water is described as 

 containing- oxygen and hydrogen, whereas really it retains 

 precisely none of the properties of either of these sub- 

 stances. Though it be admitted that it is constituted 

 of molecules, yet the molecule of water is something 

 different from its constituents ; and it is held to be a 

 crude or even unwarranted image that suggests that in it an 

 oxygen atom and two hydrogen atoms lie alongside either 

 at rest or in orbital motions. Criticism like this attaches 

 to all inferences that cannot be tested by direct sensual 

 perception. What we can know in any direct manner about 

 chemical combination is expressed merely in the laws of 

 definite and multiple proportions. Such a revision of the 

 mode of expression of our knowledge as this criticism sug- 

 gests may be useful occasionally as a stock-taking ; but the 

 misconceptions which it guards against are seldom real, 

 and indeed it makes little, if any, permanent appeal on 

 the physical side of the science. Here almost everything 

 has been constructed on the basis of dynamical ideas, — 

 those fundamental Newtonian ideas of force and inertia 

 which constitute the simplest formal scheme that admits 

 of permanence of free motions, — applied to conceptual 

 models ; such a theoretical representation is never perfect 

 or complete, but it is vivid and illuminating, and histori- 

 cally it has been progressive ; to give it up would be to 

 replace a growing system by a collection of fragments of 

 knowledge. The physicist in his own range is never 

 likely to forget that any simple piece of matter is a vast 

 interlacing, interdependent complex, which he can never 

 hope completely to disentangle or resolve : he is certain 

 that matter is of grained structure, but to him the grains 

 are very far from being mutually isolated things, — each 

 of them is actively influenced by all the others around it. 



