COMBUSTION AND CHEMICAL AFFINITY. 97 



putting a varnish, as it were, upon its surface, 

 as we varnish a picture absolutely forming 

 a substance which prevents the natural che- 

 mical affinity between the bodies from acting. 



I must now take you a little further in this 

 kind of illustration, or consideration, I would 

 rather call it, of chemical affinity. This attrac- 

 tion between different particles exists also most 

 curiously in cases where they are previously 

 combined with other substances. Here is a 

 little chlorate of potash containing the oxygen 

 which we found yesterday could be procured 

 from it ; it contains the oxygen there combined 

 and held down by its chemical affinity with 

 other things ; but still it can combine with 

 sugar, as you saw. This affinity can thus 

 act across substances, and I want you to see 

 how curiously what we call combustion acts 

 with respect to this force of chemical affinity. 

 If I take a piece of phosphorus and set fire to 

 it, and then place a jar of air over the phos- 

 phorus, you see the combustion which we are 

 having there on account of chemical affinity 

 (combustion being in all cases the result of 

 chemical affinity). The phosphorus is escaping 

 in that vapour, which will condense into a 

 H 



