62 COHESION. 



increase converts it into a gas. Is it not a 

 glorious thing for us to look at the sea, the 

 rivers, and so forth, and to know that this same 

 body in the northern regions is all solid ice and 

 icebergs, while here, in a warmer climate, it has 

 its attraction of cohesion so much diminished 

 as to be liquid water. Well, in diminishing 

 this force of attraction between the particles of 

 ice, we made use of another force, namely, that 

 of heat; and I want you now to understand that 

 this force of heat is always concerned when 

 water passes from the solid to the liquid state. 

 If I melt ice in other ways I cannot do without 

 heat (for we have the means of making ice 

 liquid without heat; that is to say, without 

 using heat as a direct cause). Suppose, for 

 illustration, I make a vessel out of this piece of 

 tinfoil [bending the foil up into the shape of 

 a dish], I am making it metallic, because I 

 want the heat which I am about to deal with 

 to pass readily through it ; and I am going 

 to pour a little water on this board, and then 

 place the tin vessel on it. Now if I put some 

 of this ice into the metal dish, and then pro- 

 ceed to make it liquid by any of the various 

 means we have at our command, it still must 



