together. 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 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[1]], 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
- ↑ Lead Pyrophorous.—This is a tartrate of lead which has been heated in a glass tube to dull redness as long as vapours are emitted. As soon as they cease to be evolved, the end of the tube is sealed, and it is allowed to cool.
