IX. 



HOW HEAT IS MADE. 



So far we have not considered how heat is produced, but have 

 merely assumed that there is a source of heat near. Now, the question 

 of how heat is made in the iirst phicc is rather a difficult one and it 

 would take us too long to explain the whole of the subject. Burning 

 something is, of course, the best known way. For instance, where 

 does the heat come from when vou strike a match? You might sup- 

 pose it due altogether to something contained in the match. But try 

 rubbing a brass button on a board, and you will find that you can 

 make it too hot to be held comfortably. By rubbing two sticks 

 together you can make them quite hot, and can understand how sav- 

 ages get fire without matches. You may not be able, like a savage, to 

 get the wood to burn, but rub hard enough and you will get another 

 interesting case of heating by friction. I shall leave you to think of 

 what it is for yourself. 



And now you should be able to explain what sometimes happens 

 when the axle of a carriaye has not been oiled, and in ci)nsc(|uence 

 grows hot and perhaps sticks fast in the wheel, or else the hub may 

 be torn from the spokes. You know that sawing produces heat, but 

 this is, of course, merely another case of rubbing. Hammering will 

 also cause a body to grow hot. Xo doubt you can make a nail quite 

 warm by hammering it on a stone. An expert smith can make a 

 piece of iron red hot by hammering. This production of heat by ham- 

 nierim,^ will perhaps help you to understand the flash of light you 

 sometimes see when the shoe on a horse's foot strikes a stone. So 

 much heat is formed that something actually burns and produces 

 lia-ht You can do the same in a dark room with a hammer and a 

 stone; and let me suggest that while you are in the dark room you 

 should try crushing a lump of sugar with a blow of the hammer; you 

 will see quite a flash of light. This is perhaps a somewhat different 

 case, for electricitv is produced, and the light you get is like a light- 

 ning flash, due to a discharge of electricity. This last remark ^^^ll 

 remind you of the production of heat and light by electricity, but 

 as we can not take space here to describe all the ways in whigh heat 

 is produced, you must be left to think of as many as you can for 

 yourself. 



X. 



WHAT IS HEAT? 



And now that you have made some experiments and observations 

 on heat without knowing what it really is, I am going to ask you to 



