122 COHESION* 



ledge of it. In order to that, there must be some- 

 thing which we can call substance, and that substance 

 must be of some extension, or measure, or bulk ; that 

 is, it must occupy space, and space in which there 

 can be no other substance at the same time. That 

 space must be of some shape orjigure, too ; and the 

 shape of the space must be exactly the same as that 

 of the body which fills it ; and the substance which 

 thus occupies space must have some consistency, in 

 order that we may know by muscular resistance 

 that it exists.; and its consistency must depend, not 

 only on what has been already noticed as its gravi- 

 tation of quantity of matter and position, by which 

 it keeps its place among other substances, but it 

 must have a consistency of its own, by means of 

 which it preserves its shape. That is called the 

 cohesion of the body, which means the tendency that 

 the still smaller bodies, of which we must suppose 

 even the very smallest that we can examine to be 

 made up, have to stick together. And this last prop- 

 erty, of which there are many varieties, has no 

 necessary connexion with universal gravitation. 

 The same bulk of wuter is, under ordinary circum- 

 stances, much heavier than cork, for cork swims on 

 the surface of water ; but water can be held up in a 

 vessel made of cork. Indeed, the cork is really 

 heavier than water; for it may be so soaked in 

 water that it will sink like a stone, and at the same 

 time be no larger than before. But the water cannot 

 possibly be heavier than itself ; and so the cork must 

 be heavier than water. Quicksilver too is much 

 heavier than glass ; and yet it can be contained in a 

 glass vessel ; but still quicksilver, though when laid 

 or poured on the table, it spreads, yet shows that it 

 has still the property of cohesion, though only to a 

 limited extent. That principle or property cannot 

 resist the action of a pound or an ounce ; but the 

 little beads of it are quite round, and they dance 

 about like small balls of polished steel. 



