HEAT AND CHEMICAL AFFINITY. 131 



changed one into the other ; and it will to-day 

 require all my care, and your care too, to make 

 this clear to your minds. I shall be obliged 

 to confine myself to one or two instances, be- 

 cause to take in the whole extent of this 

 mutual relation and conversion of forces would 

 surpass the human intellect. 



In the first place, then, here is a piece of fine 

 zinc-foil, and if I cut it into narrow strips and 

 apply to it the power of heat, admitting the 

 contact of air at the same time, you will find 

 that it burns ; and then, seeing that it burns, 

 you will be prepared to say that there is 

 chemical action taking place. You see all I 

 have to do is to hold the piece of zinc at the 

 side of the flame, so as to let it get heated, and 

 yet to allow the air which is flowing into the 

 flame from all sides to have access to it; there 

 is the piece of zinc burning just like a piece of 

 wood, only brighter. A part of the zinc is 

 going up into the air, in the form of that white 

 smoke, and part is falling down on to the table. 

 This, then, is the action of chemical affinity 

 exerted between the zinc and the oxygen of the 

 air. I will show you what a curious kind of 

 affinity this is by an experiment, which is 



K 2 



