flame (I must not talk while I make the experiment), and you will find
that it is burnt in two places, and that it is not burnt, or very little
so, in the middle; and when you have tried the experiment once or twice,
so as to make it nicely, you will be very interested to see where the heat
is, and to find that it is where the air and the fuel come together.
This is most important for us as we proceed with our subject. Air is
absolutely necessary for combustion; and, what is more, I must have you
understand that _fresh_ air is necessary, or else we should be imperfect
in our reasoning and our experiments. Here is a jar of air. I place it
over a candle, and it burns very nicely in it at first, shewing that what
I have said about it is true; but there will soon be a change. See how the
flame is drawing upwards, presently fading, and at last going out. And
going out, why? Not because it wants air merely, for the jar is as full
now as it was before; but it wants pure, fresh air. The jar is full of
air, partly changed, partly not changed; but it does not contain
sufficient of the fresh air which is necessary for the combustion of a
candle. These are all points which we, as young chemists, have to gather
up; and if we look a little more closely into this kind of action, we
shall find certain steps of reasoning extremely interesting. For instance,
here is the oil-lamp I shewed you an excellent lamp for our
experiments the old Argand lamp. I now make it like a candle [obstructing
the passage of air into the centre of the flame]; there is the cotton;
there is the oil rising up it; and there is the conical flame. It burns
poorly, because there is a partial restraint of air. I have allowed no air
to get to it, save round the outside of the flame, and it does not burn
well. I cannot admit more air from the outside, because the wick is large;
but if, as Argand did so cleverly, I open a passage to the middle of the
flame, and so let air come in there, you will see how much more
beautifully it burns. If I shut the air off, look how it smokes; and why?
We have now some very interesting points to study. We have the case of the
combustion of a candle; we have the case of a candle being put out by the
want of air; and we have now the case of imperfect combustion; and this is
to us so interesting, that I want you to understand it as thoroughly as
you do the case of a candle burning in its best possible manner. I will
now make a great flame, because we need the largest possible
illustrations. Here is a larger wick [burning turpentine on a ball of
cotton]. All these things are the same as candles, after all. If we have
larger wicks, we must have a larger supply of air, or we shall have less
perfect combustion. Look now at this black substance going up into the
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