THERMO-DYNAMICS. 45 



absolute temperature of the boiler, that is to the tempe- 

 rature measured from absolute zero, where there is no heat 

 whatever. Consequently if we indicate the absolute 

 temperature by measuring from the absolute zero, that 

 is to say, from about 273 C., and if T be the temperatuie 

 of the boiler and S the temperature of the condenser, 

 and suppose we absorb from the boiler an amount of 

 heat equal to H, then the heat which it has utilized is 



T - S 

 H X ijT-' 



There are a great number of other deductions which can 

 be made from a study of this diagram. Some of the most 

 important of these depend on the evolution of heat when 

 we compress or expand different substances ; and one of 

 the most general conclusions arrived at is this, that in the 

 case of all substances which expand with an increase of 

 temperature we shall have an increase of heat when we 

 compress them. But there are one or two substances 

 which contract when heat is added to them ; and these it 

 is found give off heat or grow cool when they are compressed. 

 Water, for example, between the temperature C. and 

 4 C., is such a body; and it was pointed out by Joule 

 that India-rubber when it is suddenly stretched instead 

 of growing cooler grows hotter, and in consequence of 

 this, it was predicted theoretically that if stretched India- 

 rubber be heated gradually, instead of lengthening it will 

 contract. This is one of the most striking applications of 

 the theory of the second law of thermo-dynamics ; but 

 perhaps the most striking one is the experiment of Prof. 

 James Thomson, which was afterwards followed by his 

 brother Sir William Thomson, in which he showed that the 

 melting point of ice is lowered when we exert great pressure 

 upon the ice. He deduced this from the fact that the water 

 expands at the moment when it solidifies. I have here an 

 arrangement by means of which perhaps I shall be able to 

 show this. But it is a troublesome experiment, and my 

 galvanometer has rather more resistance in it than is 

 desirable to show it well. I have here two wires, one of 

 iron and another of copper. There are two junctions of 

 the iron and copper wires, one of which I keep constantly 

 in a vessel containing ice and water to keep it at a constant 

 temperature, and the other junction is passed over a block 



