Considerable friction results between the wheel and the brake;--heat is produced;--the train gradually comes to a stop. Why? We have now the conversion of that force into heat which a minute ago was being used for the purpose of keeping the train a-going. Given a certain force you can have heat _or_ motion; but you cannot have heat _and_ motion with the same force in the same amount as if you had them singly. In every-day life, you cannot have your pudding and eat it. [A] I need scarcely say, that whatever is of any value in the following remarks is derived from that charming book of Professor Tyndall's, _Heat a Mode of Motion_. Heat then is generated by mechanical force; it is a mode of motion. There was an old theory that heat was material. There was heat, for instance, you were told, in this nail. Suppose I hammer it, it will get hot, and at the same time I shall reduce by hammering the bulk of the iron nail. A pint pot will not hold so much as a quart pot. The nail (you were told) cannot hold so much heat when it occupies a less bulk as it did when it occupied a larger bulk. Therefore if I reduce the bulk of the nail I squeeze out some of the heat. That was the old theory. One single experiment knocked it on the head. It was certain, that in water there is a great deal more entrapped heat--"latent heat" it was called--than there is in ice. If you take two pieces of ice and rub them together, you will find the ice melts--the solid ice changes (that is to say) into liquid water. Where did the heat come from to melt the ice? You could not get the heat _from_ the ice, because it was not there, there being admittedly more latent heat in the water than in the ice. The explanation is certain--the heat was the result of the friction. And now let me go to my hammer and nail. I wish to see whether I can make this nail hot by hammering. It is quite cold at the present time. I hope to make the nail hot enough by hammering it to fire that piece of phosphorus (Fig. 24). One or two sharp blows with the hammer suffice, and as you see the thing is done--_I_ have fired the phosphorus. But follow the precise details of the experiment. It was _I_ who gave motion to the hammer. _I_ brought that hammer on to that nail. Where did the motion go to that I gave the hammer? It went into the nail, and it is that very motion that made the nail hot, and it was that heat which lighted the phosphorus. It was _I_ who fired the phosphorus: do not be mistaken, _I_ fired the phosphorus. It was my arm that gave motion to the hammer. It was my force that was communicated to the hammer. It was _I_ who made the hammer give the motion to the nail. It was _I_ myself that fired the phosphorus. [Illustration: Fig. 24.]