spark when I make and break contact of the two poles. This is what is called an electrical torch, in which I utilize this small spark as a gas-lighter (Fig. 16). This instrument contains at its lower part a source of electricity, and if I connect the two wires that run through this long tube with the apparatus which generates the current, which I do by pressing on this button, you see a little spark is at once produced which readily sets fire to my gas-lamp. We have in this electrical torch a substitute—partial substitute, I ought to say—for the lucifer match. I think you will admit that it was with some show of reason I suggested that after all it is possible the lucifer match may not have quite so long an innings as the tinder-box. But there is another curious thing to note in these days of great scientific progress, viz. that there are signs of the old tinder-box coming to the front again. Men, I have often noticed, find it a very difficult thing to light their pipes with a match on the top of an omnibus on a windy day, and inventors are always trying to find out something that will enable them to do so without the trouble and difficulty of striking a match, and keeping the flame a-going long enough to light their cigars. And so we have various forms of pipe-lighting apparatus, of which here is one—which is nothing more than a tinder-box with its flint and steel (Fig. 17). You set to work somewhat in this way: placing the tinder (a) on the flint (b), you strike the flint with the steel (c), and—there, I have done it!—my tinder is fired by the spark. So you see there are signs, not only of the lucifer match being ousted by the applications of electricity, but of the old tinder-box coming amongst us once again in a new form.

Fig. 17.

Fig. 18.

I am now going to ask you to travel with me step by step through the operation of getting fire out of the tinder-box. The first thing I have to do is to prepare my tinder, and I told you, if you remember, that the way we made tinder was by charring pieces of linen (see Fig. 4). I told you last time what a dear old friend told me, who from practical experience is far more familiar with tinder-boxes and their working than I am, that no material was better for making tinder than an old cambric handkerchief. However, as I have no cambric handkerchief to operate upon, I must use a piece of common linen rag. I want you to see precisely what takes place. I set fire to my linen (which, by the bye, I have taken care to wash carefully so that there should be no dirt nor starch left in it), and while it is burning shut it down in my tinder-box. That is my tinder. Let us now call this charred linen by its proper name—my tinder is carbon in a state of somewhat fine subdivision. Carbon is an elementary body. An element—I do not say this is a very good definition, but it is sufficiently good for my purpose—an element is a thing from which nothing can be obtained but the element itself. Iron is an element. You cannot get anything out of iron but iron; you cannot decompose iron. Carbon is an element; you can get nothing out of carbon but carbon. You can combine it with other things, but if you have only carbon you can get nothing out of the carbon but carbon. But this carbon is found to exist in very different states or conditions. For instance, it is found in the form of the diamond. (Fig. 18 a). Diamonds consist of nothing more nor less than this simple elementary body—carbon. It is a very different form of carbon, no doubt you think, to tinder. Just let me tell you, to use a very hard word, that we call the diamond an "allotropic" form of carbon. Allotropic means an element with another form to it—the diamond is simply an allotropic form of carbon. Now the diamond is a very hard substance indeed. You know perfectly well that when the glass-cutter wants to cut glass he employs a diamond for the purpose, and the reason why glass can be cut with a diamond is because the diamond is harder than the glass. I dare say you have often seen the names of people scratched on the windows of railway-carriages, with the object I suppose that it may be known to all future occupants of these carriages that persons of a certain name wore diamond rings. Well, in addition to the diamond there is another form of carbon, which is called black-lead. Black-lead—or, as we term it, graphite—of which I have several specimens here—is simply carbon—an allotrope of carbon—the same elementary substance, notwithstanding, as the diamond. This black-lead (understand black-lead, as it is called, contains no metallic lead) is used largely for making lead-pencils. The manufacture of lead-pencils, by the bye, is a very interesting subject. Formerly they cut little pieces of black-lead out of lumps of the natural black-lead such as you see there; but now-a-days they powder the black-lead, and then compress the very fine powder into a block. There is a block of graphite or black lead, for instance, prepared by simple pressure (Fig. 18 b). The great pressure to which the powder is subjected brings these fine particles very close together, when they cohere, and form a substantial block. I will show you an experiment to illustrate what I mean. Here are two pieces of common metallic lead. No ordinary pressure would make these two pieces stick together; but if I push them together very energetically—boys would call it giving them "a shove" together—that is to say, employing considerable pressure to bring them into close contact—I have no doubt that I can make these two pieces of lead stick together—in other words, make them cohere. To cohere is not to adhere. Cohesion is the union of similar particles—like to like; adhesion is the union of dissimilar particles. Now that is exactly what is done in the preparation of the black-lead for lead-pencils. The black-lead powder is submitted to great pressure, and then all these fine particles cohere into one solid lump. The pencil maker now cuts these blocks with a saw into very thin pieces (Fig. 19 b). The next thing is to prepare the wood to receive the black-lead strips. To do this they take a piece of flat cedar wood and cut a number of grooves in it, placing one of these little strips of black-lead into each of the grooves (Fig. 19 a, which represents one of the grooves). Then having glued on the cover (Fig. 19 c), they cut it into strips, and plane each little strip into a round lead-pencil (Fig. 19 d). But what you have there as black-lead in the pencil (for this is what I more particularly wish you to remember) is simply carbon, being just the same chemical substance as the diamond. To a chemist diamond and black-lead have the same composition, being indeed the same substance. As to their money value, of course there is some difference; still, so far as chemical composition is concerned, diamonds and black-lead are both absolutely true varieties of the element carbon.

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