Page:On the various forces of nature and their relations to each other.djvu/122

118 which we must consider as if it were now for the very first time brought under our notice? What is heat? We recognise heat by its power of liquefying solid bodies and vaporising liquid bodies, by its power of setting in action, and very often overcoming, chemical affinity. Then, how do we obtain heat? We obtain it in various ways— most abundantly by means of the chemical affinity we have just before been speaking about; but we can also obtain it in many other ways. Friction will produce heat. The Indians rub pieces of wood together until they make them hot enough to take fire; and such things have been known as two branches of a tree rubbing together so hard as to set the tree 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 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