102 HEAT. 



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 chemi- 

 cal affinity will subside as soon as the stream of 

 air is interrupted. What then is this curious 

 condition of heat ? Why it 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 speak- 

 ing 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 



