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,
but they will start into activity at different degrees of heat, or under
different conditions. By applying a heated wire to them, we shall see
which will start first [touching the gun-cotton with the hot iron]. You
see the gun-cotton has gone off, but not even the hottest part of the wire
is now hot enough to fire the gunpowder. How beautifully that shews you
the difference in the degree in which bodies act in this way! In the one
case the substance will wait any time until the associated bodies are made
active by heat; but in the other, as in the process of respiration, it
waits no time. In the lungs, as soon as the air enters, it unites with the
carbon; even in the lowest temperature which the body can bear short of
being frozen, the action begins at once, producing the carbonic acid of
respiration: and so all things go on fitly and properly. Thus you see the
analogy between respiration and combustion is rendered still more
beautiful and striking. Indeed, all I can say to you