Page:Various Forces of Matter.djvu/115

Rh 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 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. 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, H 4