S FARADAY 



will burn in the common atmospheric oxygen at the ordinary 

 temperature. These are the lumps of lead which you remem- 

 ber we had the other day the two pieces which clung to- 

 gether. Now these pieces, if I take them to-day and press 

 them together, will not stick, and the reason is that they have 

 attracted from the atmosphere a part of the oxygen there 

 present, and have become coated as with a varnish by the 

 oxide of lead, which is formed on the surface by a real 

 process of combustion or combination. There you see the 

 iron burning very well in oxygen, and I will tell you the 

 reason why those scissors and that lead do not take fire while 

 they are lying on the table. Here the lead is in a lump, and 

 the coating of oxide remains on its surface, while 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(")], 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 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 at- 

 mospheric 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 

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

 a picture absolutely forming a substance which prevents 

 the natural chemical affinity between the bodies from acting. 

 I must now take you a little farther in this kind of illus- 

 tration, or consideration I would rather call it, of chemical 

 affinity. This attraction between different particles exists 

 also most curiously in cases where they are previously com- 



* Lead Pyrophorus. This is tartrate of lead which has been heated in 

 a glass tube to dull redness as long as vapors are emitted. As soon as 

 they cease to be evolved the end of the tube is sealed, and.it is allowed 

 to cool. 



