see whether that water can extinguish it. Here it is burning out of the water, and there it is burning under the water; and so it will continue until exhausted, and all by reason of the requisite amount of oxygen being contained within the substance. It is by this kind of attraction of the different particles one to the other that we are enabled to trace the laws of chemical affinity, and the wonderful variety of the exertions of these laws. Now, I want you to observe that one great exertion of this power, which is known as _chemical affinity_, is to produce HEAT and light. You know, as a matter of fact, no doubt, that when bodies burn they give out heat; but it is a curious thing that this heat does not continue--the heat goes away as soon as the action stops, and you see thereby that it depends upon the action _during the time_ it is going on. It is not so with gravitation: this force is continuous, and is just as effective in making that lead press on the table as it was when it first fell there. Nothing occurs there which disappears when the action of falling is over; the pressure is upon the table, and will remain there until the lead is removed; whereas, in the action of chemical affinity to give light and heat, they go away immediately the action is over. This lamp _seems_ to evolve heat and light continuously; but it is owing to a constant stream of air coming into it on all sides, and this work of producing light and heat by chemical affinity will subside as soon as the stream of air is interrupted. What, then, is this curious condition of heat? Why is the evolution of another power of matter, of a power new to us, and 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 hot as to scorch wood and paper, and burn a match. [Illustration: Fig. 31.]