exact line that divides wood from brass by the burning of the
paper. Well, why is that? Now all of you know that some things conduct
heat (_i. e._ carry away heat) better than other substances. For
instance, if you were to put a copper rod and a glass rod into the fire,
allowing a part of each to project, the copper rod that projects out of
the fire would soon become so very hot that you dare not touch it, owing
to the copper conducting the heat from the fire, whereas you would be
able to take hold of the projecting end of the glass rod long after the
end of the glass exposed to the fire had melted. The fact is, the copper
carries heat well, and the glass carries heat badly. Now with the
teaching of that experiment before you, you will understand, I hope, the
exact object of one or two experiments I am about to show you. Here is a
piece of coarse wire gauze--I am about to place it over the flame of
this Argand burner. You will notice that it lowers the flame for a
moment, but almost immediately the flame dashes through the gauze (Fig.
32 A). Here is another piece of gauze, not quite so coarse as the last.
I place this over the flame, and for a moment the flame cannot get
through it. There, you see it is through now, but it did not pass with
the same readiness that it did in the case of the other piece of gauze,
which was coarser. Now, when I take a piece of fine gauze, the flame
does not pass through at all until the gauze is nearly red-hot. There
is plenty of gas passing all the time. If I take a still finer gauze, I
shall find that the flame won't pass even when it is almost red-hot
(Fig. 32 B). Plenty of gas is passing through, remember, all the time,
but the flame does not pass through. Now why is it that the flame is
unable to pass? The reason is this--because the metal gauze has so
cooled the flame that the heat on one side is not sufficient to set fire
to the gas on the other side. I must have, you see, a certain
temperature to fire my gas. When therefore I experiment with a very fine
piece of gauze, where I have a good deal of metal and a large conducting
surface, there is no possibility of the flame passing. In fact, I have
so cooled the flame by the metal gauze that it is no longer hot enough
to set fire to the gas on the opposite side. I will give you one or two
more illustrations of the same fact. Suppose I put upon this gauze a
piece of camphor (camphor being a substance that gives off a heavy
combustible vap