60 FARADAY 



on fire. I do not suppose I shall set these two pieces of wood 

 on fire by friction, but I can readily produce heat enough to 

 ignite some phosphorus. [The lecturer here rubbed two 

 pieces of cedar wood strongly against each other for a min- 

 ute, and then placed on them a piece of phosphorus, which 

 immediately took fire.] And if you take a smooth metal 

 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 affinity alone, but by the pressure of air. Sup- 

 pose I take a pellet of cotton and moisten it with a 

 little ether, and put it into a glass tube (Fie. 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 pres- 

 sure, 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 vapor, 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-vapor. 



This, then, I think will be sufficient, accompanied 

 with all you have previously seen, to show you how we pro- 

 cure heat. And now for the effects of this power. We need 

 not consider many of them on the present occasion, because, 

 when you have seen its power of changing ice into water and 

 water into steam, you have seen the two principal results 

 of the application of heat. I want you now to see how it 

 expands all bodies all bodies but one, and that under limited 

 circumstances. Mr. Anderson will hold a lamp under that 

 retort, and you will see, the moment he does so, that the 

 air will issue abundantly from the neck which is under 

 water, because the heat which he applies to the air causes 

 it to expand. And here is a brass rod ( FIG. 32) which goes 

 through that hole, and fits also accurately into this gauge; 

 but if I make it warm with this spirit lamp, it will only 

 go in the gauge or through the hole with difficulty; and if 



