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.
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 catches fire directly it is exposed to the atmosphere (Fig. 21 _c_). There it is! Only powder the lead sufficiently fine,--that is to say, bring it into a state of minute subdivision,--and it fires by contact with the oxygen of the air. And now apply this. We have in our diamond the element carbon, but diamond-carbon is a hard substance, and not in a finely-divided state. We have in this tinder the same substance as the diamond, but tinder-carbon is finely divided, and it is because it is in a finely-divided condition that the carbon in our tinder-box catches fire so readily. I hope I have made that part of my subject quite clear to you. I should wish you to note that this very finely-divided carbon has rather an inclination to attract moisture. That is the reason why our tinder is so disposed to get damp, as I told you; and, as damp tinder is very difficult to light, this explains the meaning of those disrespectful words that I suggested our tinder-box had often had addressed to it in the course of its active life of service.