THE CONSTITUTION OF NATURE. 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 

 gi eater 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 attrac- 

 tion ; the attraction 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 exer- 

 cised, and also atoms between 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 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, gra- 

 vity imparts to the body a velocity of thirty-two feet a 

 second. In like manner the mutual attraction of oxygen 

 and hydrogen might be measured by the velocity im- 

 parted to the atoms in their rushing together. Of course 

 such a unit of time as a second is not here to be thought 

 of, the whole interval required by the atoms to cross the 



