[Illustration: Fig. 18.]

[Illustration: Fig. 19.]

Well now, I come to another form of carbon, called charcoal (Fig. 18
_c_). You all know what charcoal is. There is a lump of wood charcoal.
It is, as you see, very soft,--so soft indeed is it that one can cut it
easily with a knife. Graphite is not porous, but this charcoal is very
porous. But mind, whether it be diamond, or black-lead, or this porous
charcoal, each and all have the same chemical composition; they are what
we call the elementary undecomposable substance carbon. The tinder I
made a little while ago (Fig. 4), and which I have securely shut down in
my tinder-box, is carbon. It is not a diamond. It is not black-lead, but
all the same it is _carbon_--that form of porous carbon which we
generally call charcoal. Now I hope you understand the meaning of that
learned word _allotropic_. Diamond, black-lead, and tinder are
allotropic forms of carbon, just as I explained to you in my last
lecture, that the elementary body phosphorus was also known to exist in
two forms, the red and the yellow variety, each having very different
properties.

[Illustration: Fig. 20.]

Now it has been noticed when substances are in a very finely-divided
state that they often possess greater chemical activity than they have
in lump. Let me try and illustrate what I mean. Here I have a metal
called antimony, which is easily acted upon by chlorine. I will place
this lump of antimony in a jar of chlorine, and so far as you can see
very little action takes place between the metal and the chlorine. There
is an action taking place, but it is rather slow (Fig. 20 A). Now I will
introduce into the chlorine some of the same metal which I have finely
powdered. See! it catches fire immediately (Fig. 20 B). What I want you
to understand is, that although I have in both these cases precisely the
same chlorine and the same metal, nevertheless, that whilst the action
of the chlorine on the _lump_ of antimony was not very apparent, in the
case of the _powdered_ antimony the action was very energetic. Again,
there is a lump of lead (Fig. 21 _a_). You would be very much astonished
if the lead pipe that conveys the water through your houses caught fire
spontaneously; but let me tell you that, if your lead water-pipes were
reduced to a sufficiently fine powder, they would catch fire when
exposed to the air. I have some finely-powdered lead in this tube (Fig.
21 _b_), which you will notice cat