96 CHEMICAL AFFINITY. 



reason why those scissors and that lead do not 

 take fire whilst they are lying on the table. 

 Here the lead is in a lump, and the coating of 

 oxide remains on its surface, whilst there you 

 see the melted oxide is clearing itself off from 

 the iron, and allowing more and more to go on 

 burning. In this case, however [holding up a 

 small glass tube containing lead pyropHorus ( 20 )], 

 the lead has been very carefully produced in 

 fine powder, and put into a glass tube and 

 hermetically sealed so as to preserve it, and I 

 expect you will see it take fire at once. This 

 has been made about a month ago, and has 

 thus had time enough to sink down to its 

 normal temperature what you see therefore 

 is the result of chemical affinity alone. [The 

 tube was broken at the end, and the lead 

 poured out on to a piece of paper, whereupon 

 it immediately took fire.] Look, look, at the 

 lead burning ; why it has set fire to the paper ! 

 Now that is nothing more than the common 

 affinity always existing between very clean lead 

 and the atmospheric oxygen ; and the reason 

 why this iron does not burn until it is made 

 red hot, is because it has got a coating of oxide 

 about it, which stops the action of the oxygen, 



