76 FARADAY 



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 rather striking 

 when seen for the first time. I have here some iron filings 

 and gunpowder, and will mix them carefully together, with 

 as little rough handling as possible; now we will compare 

 the combustibility, so to speak, of the two. I will pour some 

 spirit of wine into a basin and set it on fire; and, having 

 our flame, I will drop this mixture of iron filings and gun- 

 powder through it, so that both sets of particles will have 

 an equal chance of burning. And now tell me which of 

 them it is that burns? You see a plentiful combustion of 

 the iron filings ; but I want you to observe that, though they 

 have equal chances of burning, we shall find that by far the 

 greater part of the gunpowder remains untouched; I have 

 only to drain off this spirit of wine, and let the powder 

 which has gone through the flame dry, which it will do in 

 a few minutes, and I will then test it with a lighted match. 

 So ready is the iron to burn, that it takes, under certain 

 circumstances, even less time to catch fire than gunpowder. 

 [As soon as the gunpowder was dry, Mr. Anderson handed 

 it to the lecturer, who applied a lighted match to it, when 

 a sudden flash showed how large a proportion of gunpowder 

 had escaped combustion when falling through the flame of 

 alcohol.] 



These are all cases of chemical affinity, and I show them, 

 to make you understand that we are about to enter upon 

 the consideration of a strange kind of chemical affinity, and 

 then to see how far we are enabled to convert this force of 

 affinity into electricity or magnetism, or any other of the 

 forces which we have discussed. Here is some zinc (I keep 

 to the metal zinc, as it is very useful for our purpose), and 

 I can produce hydrogen gas by putting the zinc and sulphuric 

 acid together, as they are in that retort; there you see the 

 mixture which gives us hydrogen the zinc is pulling the 

 water to pieces and setting free hydrogen gas. Now we 

 have learned by experience that if a little mercury is spread 

 over that zinc, it does not take away its power of decom- 

 posing the water, but modifies it most curiously. See how 



