36 LECTURES TO SCIENCE TEACHERS. 



that is to say, if a weight of 1 Ib. be allowed to fall from 

 a height of 772 feet, it would generate heat when it 

 stopped sufficient to raise 1 Ib. of water 1 Fahr. These 

 experiments were very important because they were 

 entirely of a new order. Rumford and Davy in previous 

 times had almost as clearly as Joule pointed out that theie 

 must be a mechanical equivalent of heat although they 

 gave no exact experiments proving that this was the case ; 

 but philosophers up to the time of Joule had never admitted 

 that the friction of liquids or the friction of gases could 

 produce these differences of temperature, as was known to 

 be the case with the friction of solids. Rumford 's cele- 

 brated experiment consisted in measuring the increase of 

 temperature in the boring of a cannon, but people who 

 supported the theory that heat is matter said that the 

 material of which the cannon was composed changed its 

 state when it was bored, and consequently that there might 

 be some evolution of latent heat. But in the case of the 

 water which was employed by Joule there is not the 

 slightest doubt that the water at the end of the experiment 

 is in precisely the same condition as at the beginning, with 

 the exception that it is so many degrees hotter. 



Now in order to show you that this is really true with 

 respect to gases, which is perhaps less easy to grasp, I will 

 show you experiments which Vill illustrate the action of 

 gases in producing heat by friction. Here I have a delicate 

 galvanometer which is in connection by these wires with a 

 thermopile, the most delicate means of measuring tempera- 

 ture which we possess. As soon as I heat one side of this 

 thermopile the needle is driven away in one direction, and 

 if I were to cool it by putting a block of ice near it, the 

 needle would move in the opposite direction. Now if I 

 take a pair of bellows and blow gently on the face of 

 the thermopile there will be sufficient friction of the 

 air to produce a sensible motion of tha,t needle. You 

 see as I blow the face of the pile is gradually warmed 

 and the needle moves to one side, showing that there is a 

 certain amount of heat generated by the friction of the 

 air against the pile. In this case the work is done at the 

 point of contact between the air and the pile ; but if I 

 employ air which is intensely compressed, such as that 

 which is inclosed in this vessel, so soon as I let the air 



