within us. When we burn a candle, the attraction of the different parts
one to the other is going on. Here it is going on in this case of the
lead; and it is a beautiful instance of chemical affinity. If the products
of combustion rose off from the surface, the lead would take fire, and go
on burning to the end; but you remember that we have this difference
between charcoal and lead--that, while the lead can start into action at
once, if there be access of air to it, the carbon will remain days, weeks,
months, or years. The manuscripts of Herculaneum were written with
carbonaceous ink, and there they have been for 1,800 years or more, not
having been at all changed by the atmosphere, though coming in contact
with it under various circumstances. Now, what is the circumstance which
makes the lead and carbon differ in this respect? It is a striking thing
to see that the matter which is appointed to serve the purpose of fuel
_waits_ in its action: it does not start off burning, like the lead and
many other things that I could shew you; but which I have not encumbered
the table with; but it waits for action. This waiting is a curious and
wonderful thing. Candles--those Japanese candles, for instance--do not
start into action at once, like the lead or iron (for iron finely divided
does the same thing as lead), but there they wait for years, perhaps for
ages, without undergoing any alteration. I have here a supply of coal-gas.
The jet is giving forth the gas, but you see it does not take fire--it
comes out into the air, but it waits till it is hot enough before it
burns. If I make it hot enough, it takes fire. If I blow it out, the gas
that is issuing forth waits till the light is applied to it again. It is
curious to see how different substances wait--how some will wait till the
temperature is raised a little, and others till it is raised a good deal.
I have here a little gunpowder and some gun-cotton; even these things
differ in the conditions under which they will burn. The gunpowder is
composed of carbon and other substances, making it highly combustible; and
the gun-cotton is another combustible preparation. They are both waiting,