THE CONSTITUTION OF NATURE 13 



tary parts. We learn, for example, that the water of our 

 rivers is formed by the union, in definite proportions, of 

 two gases, oxygen and hydrogen. We know how to bring 

 these constituents together, so as to form water: we also 

 know how to analyze the water, and recover from it its 

 two constituents. So, likewise, as regards the solid por- 

 tions of the earth. Our chalk hills, for example, are 

 formed by a combination of carbon, oxygen, and calcium. 

 These are the so-called elements the union of which, in 

 definite proportions, has resulted in the formation of 

 chalk. The flints within the chalk we know to be a com- 

 pound of oxygen and silicium, called silica; and our ordi- 

 nary clay is, for the most part, formed by the union of 

 silicium, oxygen, and the well-known light metal, alu- 

 minium. By far the greater portion of the earth's crust 

 is compounded of the elementary substances mentioned in 

 these few lines. 



The principle of gravitation has been already described 

 as an attraction which every particle of matter, however 

 small, exerts on every other particle. With gravity there 

 is no selection; no particular atoms choose, by preference, 

 other particular atoms as objects of attraction; the attrac- 

 tion of gravitation is proportional simply to the quantity 

 of the attracting matter, regardless of its quality. But in 

 the molecular world which we have now entered matters 

 are otherwise arranged. Here we have atoms between 

 which a strong attraction is exercised, and also atoms be- 

 tween which a weak attraction is exercised. One atom 

 can jostle another out of its place, in virtue of a superior 

 force of attraction. But, though the amount of force ex- 

 erted varies thus from atom to atom, it is still an attrac- 

 tion of the same mechanical quality, if I may use the term, 



