We were engaged in our last lecture in considering the various methods that have been adopted from early times for obtaining fire, and we left off at the invention of the lucifer match. I ventured to hint at the conclusion of my last lecture, that the tinder-box had something to say to the lucifer match, by way of suggestion, that just as the lucifer match had ousted it, so it was not impossible that something some day might oust the lucifer match. Electricians have unlimited confidence (I can assure you) in the unlimited applications of electricity:—they believe in their science. Now one of the effects of electricity is to cause a considerable rise of temperature in certain substances through which the electrical current is passed. Here is a piece of platinum wire, for example, and if I pass an electrical current through it, you see how the wire glows (Fig. 14). If we were to pass more current through it, which I can easily do, we should be able to make the platinum wire white hot, in which condition it would give out a considerable amount of light. There is the secret of those beautiful incandescent glow lamps that you so often see now-a-days (Fig. 15). Instead of a platinum wire, a fine thread of carbon is brought to a very high temperature by the passage through it of the electrical current, in which condition it gives out light. All that you have to do to light up is to connect your lamp with the battery. The reign of the match, as you see, so far as incandescent electric lamps are concerned, is a thing of the past. We need no match to fire it. Here are various forms of these beautiful little lamps. This is, as you see, a little rosette for the coat. Notice how I can turn the minute incandescent lamp, placed in the centre of the rose, off or on at my pleasure. If I disconnect it with the battery, which is in my pocket, the lamp goes out; if I connect it with my battery the lamp shines brilliantly. This all comes by "switching it on" or "switching it off," as we commonly express the act of connecting or disconnecting the lamp with the source of electricity.

Here is another apparatus to which I desire to call your attention. If I take a battery such as I have here—a small galvanic battery of some ten cells—you will see a very little 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.

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.

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.

But to proceed. What do I want now? I want a spark to fire my tinder. A spark is enough. Do you remember the motto of the Royal Humane Society? Some of my young friends can no doubt translate it, "Lateat scintilla forsan"—perchance a spark may lie hid. If a person rescued from drowning has but a spark of life remaining, try and get the spark to burst into activity. That is what the motto of that excellent society means. How am I to get this spark from the flint and steel to set fire to my tinder? I take the steel in one hand, as you see, and I set to work to strike it as vehemently as I can with the flint which I hold in the other (Fig. 3 A B). Spark follows spark. See how brilliant they are! But I want one spark at least to fall on my tinder. There, I have succeeded, and it has set fire to my tinder. One spark was enough. The spark was obtained by the collision of the steel and flint. The sparks produced by this striking of flint against steel were formerly the only safe light the coal-miner had to light him in his dark dreary work of procuring coal. Here is the flint and steel lamp which originally belonged to Sir Humphry Davy (Fig. 22). The miners could not use candles in coal-mines because that would have been dangerous, and they were driven to employ an apparatus consisting of an iron wheel revolving against a piece of flint for the purpose of getting as much light as the sparks would yield. This instrument has been very kindly lent to me by Professor Dewar. I will project a picture of the apparatus on the screen, so that those at a distance may be better able to see the construction of the instrument.

And now follow me carefully. I take the steel and the flint, and striking them together I get sparks. I want you to ask yourselves, Where do the sparks come from? Each spark is due to a minute piece of iron being knocked off the steel by the blow of flint with steel. Note the precise character of the spark. Let me sprinkle some iron filings into this large gas flame. You will notice that the sparks of burning iron filings are very similar in appearance to the spark I produce by the collision of my flint and steel.

But now I want to carry you somewhat further in our story. It would not do for me simply to knock off a small piece of iron; I want when I knock it off that it should be red-hot. Stay for a moment and think of this—iron particles knocked off—iron particles made red-hot. All mechanical force generates heat.A You remember, in my last lecture, I rubbed together some pieces of wood, and they became sufficiently hot to fire phosphorus. On a cold day you rub your hands together to warm them, and the cabmen buffet themselves. It is the same story—mechanical force generating heat! The bather knows perfectly well that a rough sea is warmer than a smooth sea. Why?—because the mechanical dash of the waves has been converted into heat. Let me remind you of the familiar phrase, "striking a light," when I rub the match on the match-box. "Forgive me urging such simple facts by such simple illustrations and such simple experiments. The facts I am endeavouring to bring before you are illustrations of principles that determine the polity of the whole material universe." Friction produces heat. Here is a little toy (cracker) that you may have seen before (Fig. 23). It is scientific in its way. A small quantity of fulminating material is placed between two pieces of card on which a few fragments of sand have been sprinkled (Fig. 23 a). The two ends of the paper (b b) are pulled asunder. The friction produces heat, the heat fires the fulminate, and off it goes with a crack. And now put this question to yourselves, What produced the friction? Force. What is more, the amount of heat produced is the exact measure of the amount of force used. Heat is a form of force. I must urge you to realize precisely this energy of force. When you sharpen a knife you put oil upon the hone. Why?—When the carpenter saws a piece of wood he greases the saw. Why?—When you travel by train you see the railway-porter running up and down the platform with a box of yellow grease with which he greases the wheels. Why?—The answer to these questions is not far to seek—it is because you want your knife sharpened; it is because you want the saw to cut; it is because you want the train to travel. The carpenter finds sawing hard work, and he does not want the force of the muscles of his arm—his labour, in short—to be converted into heat, and so he greases the saw, knowing that the more completely he prevents friction, the more wood he will cut. It is the force of steam that makes the engine travel. Steam costs money. The engine-driver does not want that steam-force to be converted into heat, because every degree of heat produced means diminished speed of his train; and so the porter greases the wheels. But as you approach the station the train must be stopped. The steam is turned off, and the guard puts on what he calls "the brake." What is the brake? It is a piece of wood so constructed and placed that it can be made to press upon the wheel. Considerable friction results between the wheel and the brake;—heat is produced;—the train gradually comes to a stop. Why? We have now the conversion of that force into heat which a minute ago was being used for the purpose of keeping the train a-going. Given a certain force you can have heat or motion; but you cannot have heat and motion with the same force in the same amount as if you had them singly. In every-day life, you cannot have your pudding and eat it.

A I need scarcely say, that whatever is of any value in the following remarks is derived from that charming book of Professor Tyndall's, Heat a Mode of Motion.