38 LECTURES TO SCIENCE TEACHERS. 



great mechanical force. Or, again, when a body changes its 

 condition from the liquid to the solid form, a considerable 

 amount of work is done, and that is shown by a change in 

 the temperature. For example, we may employ such a 

 solution as this of glauber salts which is supersaturated, 

 that is to say, it contains more salt than is natural to it in 

 its present condition, and the reason why it does not 

 crystallise is that the solution is in a sort of passive state ; 

 it is ready to crystallise, but it has received no inducement 

 to crystallise in one direction more than in another. If, 

 however, I introduce into the liquid a small piece of the 

 crystal itself, then crystals will be formed about this piece, 

 and there will be a sudden solidification of the contents of 

 the vessel. Now when this substance changes from the 

 liquid to the solid state there is a contraction of the mass 

 which is equivalent to a variation of what I may call the 

 potential energy of the molecules of this mass. They 

 are brought closer into contact with an evolution of heat, 

 just as you may imagine an evolution of heat if two 

 planets meeting each other were suddenly brought to a 

 stop by their collision. This will be shown by the move- 

 ments of the needle as the liquid solidifies by crystallisation. 

 So it is always; whenever there is that change of state 

 there is a waste or gain of heat from the substance. 



Now I must pass from this portion of the lecture as 

 shortly as possible, which is an enunciation of the first 

 law of thermo-dynamics which is illustrated by these 

 experiments we have seen, and which was the great labour 

 effected by Joule ; but I recommend you all to read the first 

 account of his work which he published in the Philosophical 

 Transactions for 1850. 



But now I will pass on and try to explain some of the 

 very remarkable facts which have been brought to light 

 by the study of what is called the second law of thermo- 

 dynamics. 



I suppose that we have a piston working in a cylinder 

 and gas, then if we exert a pressure upon the piston we 

 reduce the volume of gas, and the greater the pressure the 

 less is the volume occupied by the gas. If we increase the 

 pressure very slowly so that the temperature remains 

 uniform we shall find that the substance will have its 

 condition in every state represented by such curves as 



