24 WELLS'S NATURAL PHILOSOPHY. 



or other bodies, thus demonstrating their existence, though invisible, and also 

 their impenetrability. 



36. Most substances can be made to assume 

 circumstances succcssivelj the foi'm of a solid, a liquid, or a 

 Bumeth°e^rm gas. lu solids, the attractive force is the 

 Liquid," or' a strongcst ; the particles keep their places, and 

 ^^^ the solid retains its form. But if we heat the 

 golid to a sufficient degree, as, for example, a piece of iron, 

 we gradually destroy the attractive force, and the repul- 

 sive force increases ; the particles become movable, and we 

 say the body melts, or becomes a liquid. In liquids, the 

 attractive and rejDulsive forces are nearly balanced, but if 

 we supply an additional quantity of heat, we destroy the 

 attractive force altogether, and the hquid changes to a 

 gas, in which the repulsive force prevails, and the particles 

 tend to fly off from each other. By the withdrawal of 

 heat {i. e., by the application of cold), we can diminish, or 

 destroy the repulsive force, and allow the attractive force 

 to again predominate. 



Thus steam, when cooled, becomes a 

 Fig. 2. liquid, water ; and this in turn, by the 



withdrawal of an additional amount of 

 heat, becomes a solid, ice. 



The power of the repulsive force is strik- 

 ingly illustrated by the conversion of water 

 into steam. In a cubic inch of water con- 

 verted into steam, the particles will repel 

 each other to such an extent, that the space 

 occupied by the steam wUl be 1700 times 

 greater than that occupied by the water. 

 Fig. 2 illustrates the comparative difference 

 between the bulk of steam and the bulk of 

 water. 



37. The term Fluid is applied to those 

 bodies whose particles move easily among 



themselves. It is used to designate either liquids or 

 gases. . ^ 



What are the ^8. Wc distiuguish FOUR kiuds of molecular 

 moLcuu/a^^ attraction, or attraction acting upon the par- 

 traction? ticlcs of bodics at insensible distances. These 



