THE CONSTITUTION OF NATUEE. 9 



resulted in the formation of chalk. The flints within 

 the chalk we know to be a compound of oxygen and 

 silicium, called silica; and our ordinary clay is, for the 

 most part, formed by the union of silicium, oxygen, and 

 the well-known light metal, aluminium. 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 de- 

 scribed 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 at- 

 traction; the attraction of gravitation is proportional 

 simply to the quantity of the attracting matter, regard- 

 less 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 between which a weak attrac- 

 tion 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 exerted varies thus 

 from atom to atom, it is still an attraction of the same 

 mechanical quality, if I may use the term, as that of 

 gravity itself. Its intensity might be measured in the 

 same way, namely by the amount of motion which it 

 can generate in a certain time. Thus the attraction of 

 gravity at the earth's surface is expressed by the number 

 32; because, when acting freely on a body for a second 

 of time, gravity imparts to the body a velocity of thirty- 

 two feet a second. In like manner the mutual attrac- 

 tion of oxygen and hydrogen might be measured by the 

 velocity imparted to the atoms in their rushing to- 

 gether. Of course such a unit of time as a second is not 

 here to be thought of, the whole interval required by 



