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 at the end of these
lectures (for we must come to an end at one time or other) is to express a
wish that you may, in your generation, be fit to compare to a candle; that
you may, like it, shine as lights to those about you; that, in all your
actions, you may justify the beauty of the taper by making your deeds
honourable and effectual in the discharge of your duty to your fellow-men.
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