VARIOUS METHODS OF OBTAINING HEAT. 103 



pieces of cedar wood strongly against each other 

 for a minute, and then placed on them a piece 

 of phosphorus, which immediately took 

 fire.] And if you take a smooth metal ig ' 3I ' 

 button stuck on a cork, and rub it on 

 a piece of soft deal wood, you will 

 make it so hot as to scorch wood and 

 paper, and burn a match. 



I am now going to show you that we 

 can obtain heat not by chemical affi- 

 nity alone, but by the pressure of air. 

 Suppose I take a pellet of cotton and 

 moisten it with a little ether, and put 

 it into a glass tube (fig. 31), and then 

 take a piston and press it down suddenly, I expect 

 I shall be able to burn a little of that ether in 

 the vessel. It wants a suddenness of pressure, 

 or we shall not do what we require. [The 

 piston was forcibly pressed down, when a flame 

 due to the combustion of the ether was visible 

 in the lower part of the syringe.] All we want 

 is to get a little ether in vapour, and give fresh 

 air each time, and so we may go on again and 

 again getting heat enough by the compression 

 of air to fire the ether-vapour. 



This, then, I think, will be sufficient, accom- 



H 4 



