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
atmosphere; there is a regular stream of it. I have provided means to
carry off the imperfectly burned part, lest it should annoy you. Look at
the soots that fly off from the flame: see what an imperfect combustion it
is, because it cannot get enough air. What, then, is happening? Why,
certain things which are necessary to the combustion of a candle are