CHEMICAL AFFINITY AND ELECTRICITY. 133 



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 re- 

 tort ; 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 mer- 

 cury is spread over that zinc, it does not take 

 away its power of decomposing the water, but 

 modifies it most curiously. See how that mix- 

 ture is now boiling, but when I add a little 

 mercury to it the gas ceases to come off. We 

 have now scarcely a bubble of hydrogen set 

 free, so that the action is suspended for the 

 time. We have not destroyed the power of 

 chemical affinity, but modified it in a wonderful 



K 3 



