98 FARADAY 



water increase in their mutual attraction and become ice ; and 

 above a certain temperature the attraction decreases and 

 the water becomes steam. And exactly the same thing hap- 

 pens with platinum, and nearly every substance in nature; 

 if the temperature is increased to a certain point it becomes 

 liquid and a farther 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 can not 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 [bend- 

 ing 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 proceed to make it liquid by any of the 

 various means we have at our command, it still must take 

 the necessary quantity of heat from something, and in this 

 case it will take the heat from the tray, and from the water 

 underneath, and from the other things round about. Well, a 

 little salt added to the ice has the power of causing it to 

 melt, and we shall very shortly see the mixture become quite 

 fluid, and you will then find that the water beneath will 

 be frozen frozen because it has been forced to give up 

 that heat which is necessary to keep it in the liquid state 

 to the ice on becoming liquid. I remember once, when I 

 was a boy, hearing of a trick in a country ale-house: the 

 point was how to melt ice in a quart pot by the fire and 

 freeze it to the stool. Well, the way they did it was this: 

 they put some pounded ice in a pewter pot, and added some 

 salt to it, and the consequence was that when the salt was 



