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.
Fig. 21.
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.
Fig. 22.
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.
Fig. 23.
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.