Understand then why my sulphur match wanted some time and some coaxing before it caught fire, viz. to change this solid sulphur into gaseous sulphur.

[Ill.u.s.tration: Fig. 28.]

But let us go a step further: why must the solid sulphur be converted into a gas? We want a flame, and whenever we have flame it is absolutely necessary that we should have a gas to burn. You cannot have flame without you have gas. Let me endeavour to ill.u.s.trate what I mean. I pour into this flask a small quant.i.ty of ether, a liquid easily converted into a gas. If I apply a lighted taper to the mouth of the flask, no gas, or practically none, being evolved at the moment, nothing happens.

But I will heat the ether so as to convert it into a gas. And now that I have evolved a large quant.i.ty of ether gas, when I apply a lighted taper to the mouth of the flask I get a large flame (Fig. 29). There it is! The more gas I evolve (that is, the more actively I apply the heat) the larger is the flame. You see it is a very large flame now. If I take the spirit lamp away, the production of gas grows less and less, until my flame almost dies out; but you see if I again apply my heat and set more gas free, I revive my flame. I want you to grasp this very important fact, upon which I cannot enlarge further now, that given flame, I must have a gas to burn, and therefore heat as a power is needed before I can obtain flame.

[Ill.u.s.tration: Fig. 29.]

Well, you ask me, is that true of all flame? Where is the gas, you say, in that candle flame? Think for a moment of the science involved in lighting a candle. What am I doing when I apply a lighted match to this candle? The first thing I do is to melt the tallow, the melted tallow being drawn up by the capillarity of the wick. The next thing I do is to convert the liquid tallow into a gas. This done, I set fire to the gas.

I don't suppose you ever thought so much was involved in lighting a candle. My candle is nothing more than a portable gas-works, similar in principle to the gas-works from which the gas that I am burning here is supplied. Whether it is a lamp, or a gas-burner, or a candle, they are all in a true sense gas-works, and they all pre-suppose the application of heat to some material or another for the purpose of forming a gas which will burn.

[Ill.u.s.tration: Fig. 30.]

Before I pa.s.s on, I want to refer to the beautiful burner that I have here. It is the burner used by the Whitechapel stall-keepers on a Sat.u.r.day night (Fig. 30). (Fig. _a_ is an enlarged drawing of the burner.) Just let me explain the science of the Whitechapel burner.

First of all you will see the man with a funnel filling this top portion with naphtha (_c_). Here is a stop-c.o.c.k, by turning which he lets a little naphtha run down the tube through a very minute orifice into this small cup at the bottom of the burner (_a_). This cup he heats in a friend's lamp, thereby converting the liquid naphtha, which runs into the cup, into a gas. So soon as the gas is formed--in other words, so soon as the naphtha has been sufficiently heated--the naphtha gas catches fire, the heat being then sufficient to maintain that little cup hot enough to keep up a regular supply of naphtha gas. When the lamp does not burn very well, you will often see the man poking it with a pin. The carbon given off from the naphtha is very disposed to choke up the little hole through which the naphtha runs into the cup, and the costermonger pushes a pin into the little hole to allow the free pa.s.sage of the naphtha. That, then, is the mechanism of this beautiful lamp of the Whitechapel traders, known as Halliday's lamp.

Now I go to another point: having obtained the gas, I must set fire to it. It is important to note that the temperature required to set fire to different gases varies with the gas. For instance, I will set free in this bottle a small quant.i.ty of gas, which fires at a very low temperature. It is the vapour of carbon disulphide. See, I merely place a hot rod into the bottle, and the gas fires at once. If I put a hot rod into this bottle of coal gas, no such effect results, since coal gas requires a very much higher temperature to ignite it than bisulphide of carbon gas. I want almost--not quite--actual flame to fire coal gas. But here is another gas, about which I may have to say something directly, called marsh gas (the gas of coal-mines). This requires a much higher temperature than even coal gas to fire it. I want you to understand that although all gases require heat to fire them, different gases ignite at very different temperatures. Bisulphide of carbon gas, _e. g._, ignites at a very low temperature, whilst marsh gas requires a very high temperature indeed for its ignition. You will see directly that this is a very important fact. Sulphur gas ignites fortunately at a fairly low temperature, and that is why sulphur is so useful an addition to the wood splint by which to get fire out of the tinder-box.

[Ill.u.s.tration: Fig. 31.]

And here I wish to make a slight digression in my story. I will show you an experiment preparatory to bringing before you the fact I am anxious now to make clear. I have before me a tube, one half of which is bra.s.s and the other half wood. I have covered the tube, as you see, with a tightly-fitting piece of white paper. The whole tube, wood and bra.s.s, has been treated in exactly the same manner. Now I will set fire to some spirit in the trough I have here, and expose the entire tube to the action of the flame. Notice this very curious result, viz. that the paper covering the bra.s.s portion of the tube does not catch fire, whereas the paper covering the wood is rapidly consumed (Fig. 31). You see the exact line that divides wood from bra.s.s by the burning of the paper. Well, why is that? Now all of you know that some things conduct heat (_i. e._ carry away heat) better than other substances. For instance, if you were to put a copper rod and a gla.s.s rod into the fire, allowing a part of each to project, the copper rod that projects out of the fire would soon become so very hot that you dare not touch it, owing to the copper conducting the heat from the fire, whereas you would be able to take hold of the projecting end of the gla.s.s rod long after the end of the gla.s.s exposed to the fire had melted. The fact is, the copper carries heat well, and the gla.s.s carries heat badly. Now with the teaching of that experiment before you, you will understand, I hope, the exact object of one or two experiments I am about to show you. Here is a piece of coa.r.s.e wire gauze--I am about to place it over the flame of this Argand burner. You will notice that it lowers the flame for a moment, but almost immediately the flame dashes through the gauze (Fig.

32 A). Here is another piece of gauze, not quite so coa.r.s.e as the last.

I place this over the flame, and for a moment the flame cannot get through it. There, you see it is through now, but it did not pa.s.s with the same readiness that it did in the case of the other piece of gauze, which was coa.r.s.er. Now, when I take a piece of fine gauze, the flame does not pa.s.s through at all until the gauze is nearly red-hot. There is plenty of gas pa.s.sing all the time. If I take a still finer gauze, I shall find that the flame won't pa.s.s even when it is almost red-hot (Fig. 32 B). Plenty of gas is pa.s.sing through, remember, all the time, but the flame does not pa.s.s through. Now why is it that the flame is unable to pa.s.s? The reason is this--because the metal gauze has so cooled the flame that the heat on one side is not sufficient to set fire to the gas on the other side. I must have, you see, a certain temperature to fire my gas. When therefore I experiment with a very fine piece of gauze, where I have a good deal of metal and a large conducting surface, there is no possibility of the flame pa.s.sing. In fact, I have so cooled the flame by the metal gauze that it is no longer hot enough to set fire to the gas on the opposite side. I will give you one or two more ill.u.s.trations of the same fact. Suppose I put upon this gauze a piece of camphor (camphor being a substance that gives off a heavy combustible vapour when heated), and then heat it, you see the camphor gas burning on the under side of the gauze, but the camphor gas on the upper side is not fired (Fig. 33). Plenty of camphor gas is being given off, but the flame of the burning camphor on the under side is not high enough to set fire to the camphor gas on the upper side, owing to the conducting power of the metal between the flame and the upper gas.

There is one other experiment I should like to show you. Upon this piece of metal gauze I have piled up a small heap of gunpowder. I will place a spirit-lamp underneath the gunpowder, as you see I am now doing, and I don't suppose the gunpowder will catch fire. I see the sulphur of the gunpowder at the present moment volatilizing, but the flame, cooled by the action of the metal, is not hot enough to set fire to the gunpowder.

[Ill.u.s.tration: Fig. 32.]

[Ill.u.s.tration: Fig. 33.]

I showed you the steel and flint lamp--if I may call it a lamp--used by coal-miners at the time of Davy (Fig. 22). Davy set to work to invent a more satisfactory lamp than that, and the result of his experiments was the beautiful miner's lamp which I have here (Fig. 34). I regard this lamp with considerable affection, because I have been down many a coal-mine with it. This is the coal-miner's safety-lamp. The old-fashioned form of it that I have here has been much improved, but it ill.u.s.trates the principle as well as, if not better than, more elaborate varieties. It is simply an oil flame covered with a gauze shade, exactly like that gauze with which I have been experimenting. I will allow a jet of coal gas to play upon this lamp, but the gas, as you see, does not catch fire. You will notice the oil flame in the lamp elongates in a curious manner. The flame of the lamp cooled by the gauze is not hot enough to set fire to the coal gas, but the appearance of the flame warns the miner, and tells him when there is danger. And that is the explanation of the beautiful miner's safety-lamp invented by Sir Humphry Davy.

[Ill.u.s.tration: Fig. 34.]

Now let me once more put this fact clearly before you, that whether it is the gas flame or our farthing rushlight, whether it is our lamp or our lucifer match, if we have a flame we must have a gas to burn, and having a gas, we must heat it to, and maintain it at, a certain temperature. We have now reached a point where our tinder-box has presented us with flame. A flame is indeed the consummated work of the tinder-box.

[Ill.u.s.tration: Fig. 35.]

[Ill.u.s.tration: Fig. 36.]

Just let me say a few words about the grand result--the consummated work of the tinder-box. A flame is a very remarkable thing. It looks solid, but it is not solid. You will find that the inside of a flame consists of unburnt gas--gas, that is to say, not in a state of combustion at all. The only spot where true combustion takes place is the outer covering of the flame. I will try to show you some experiments ill.u.s.trating this. I will take a large flame for this purpose. Here is a piece of gla.s.s tube which I have covered with ordinary white paper.

Holding the covered gla.s.s tube in our large flame for a minute or two, you observe I get two rings of charred paper, corresponding to the outer envelope of the flame, whilst that portion of the paper between the black rings has not even been scorched, showing you that it is only the outer part of the flame that is burning (Fig. 35). The heat of the flame is at that part where, as I said before, the combustible gases come into contact--into collision with the atmosphere. So completely is this true, that if I take a tube, such as I have here, I can easily convey the unburnt gas in the centre of the flame away from the flame, and set fire to it, as you see, at the end of the gla.s.s tube a long distance from the flame (Fig. 36). I will place in the centre of my flame some phosphorus which is at the present moment in a state of active burning, and observe how instantly the combustion of the phosphorus ceases so soon as it gets into the centre of the flame. The crucible which contains it is cooled down immediately, and presents an entirely different appearance within the flame to what it did outside the flame.

It is a curious way, perhaps you think, to stop a substance burning by putting it into a flame. Indeed I can put a heap of gunpowder inside a flame so that the outer envelope of burning gas does not ignite it (Fig.

37). There you see a heap of gunpowder in the centre of our large flame.

The flame is so completely hollow that even it cannot explode the powder.

[Ill.u.s.tration: Fig. 37.]

[Ill.u.s.tration: Fig. 38.]

I want you, if you will, to go a step further The heat of the flame is due, as I explained in my last lecture, to the clashing of molecules.

But what is the light of my candle and gas due to? The light is due to the solid matter in the flame, brought to a state of white heat or incandescence by the heat of the flame. The heat is due to the clashing of the particles, the light is due to the heated solid matter in the flame. Let me see if I can show you that. I am setting free in this bottle some hydrogen, which I am about to ignite at the end of this piece of gla.s.s tube (Fig. 38 A). I shall be a little cautious, because there is danger if my hydrogen gets mixed with air. There is my hydrogen burning; but see, it gives little or no light. But this candle flame gives light. Why? The light of the candle is due to the intensely heated solid matter in the flame; the absence of light in the hydrogen flame depends on the absence of solid matter. Let me hold clean white plates over both these flames. See the quant.i.ty of black solid matter that I am able to collect from this candle flame (Fig. 38 B). But my hydrogen yields me no soot or solid matter whatsoever (Fig. 38 A). The plate remains perfectly clean, and only a little moisture collects upon it.

The light that candle gives depends upon the solid matter in the flame becoming intensely heated. If what I say be true, it follows that if I take a flame which gives no light, like this hydrogen flame (Fig. 39 A), and give it solid particles, I ought to change the non-luminous flame into a luminous one. Let us see whether this be so or not. I have here a gla.s.s tube containing a little cotton wadding (Fig. 39 B _a_), and I am about to pour on the wadding a little ether, and to make the hydrogen gas pa.s.s through the cotton wadding soaked with ether before I fire it.

And now if what I have said is correct, the hydrogen flame to which I have imparted a large quant.i.ty of solid matter ought to produce a good light, and so it does! See, I have converted the flame which gave no light (Fig. 39 A) into a flame which gives an excellent light merely by incorporating solid matter with the flame (Fig. 39 B). What is more, the amount of light that a flame gives depends upon the amount or rather the number of solid particles that it contains. The more solid particles there are in the flame, the greater is the light. Let me give you an ill.u.s.tration of this. Here is an interesting little piece of apparatus given to my predecessor in the chair of chemistry at the London Hospital by the Augustus Harris of that day. It is one of the torches formerly used by the pantomime fairies as they descended from the realms of the carpenters. I have an alcohol flame at the top of the torch which gives me very little light. Here, you see, is an arrangement by which I can shake a quant.i.ty of solid matter (lycopodium) into the non-luminous alcohol flame. You will observe what a magnificently luminous flame I produce (Fig. 40).

[Ill.u.s.tration: Fig. 39.]

[Ill.u.s.tration: Fig. 40.]

I have told you that the light of a flame is due to solid matter in the flame;[B] further, that the amount of light is due to the amount of solid matter. And now I want to show you that the kind of light is due to the kind of solid matter in the flame. Here are some pieces of cotton wadding, which I am about to saturate with alcoholic solutions of different kinds of solid matter. For instance, I have in one bottle an alcoholic solution of a lithium salt, in another of a barium, in a third of a strontium, and so on. I will set fire to all these solutions, and you see how vastly different the colours are, the colour of the flames being dependent on the various forms of solid matter that I have introduced into them.

[B] I have not forgotten Frankland's experiments on this subject, but the lectures did not admit of dealing with exceptional cases.

Thus I have shown you that the heat of our flame is due to the clashing of the two gases, and the light of the flame to the solid matter in the flame, and the kind of light to the kind of solid matter.

Well, there is another point to which I desire to refer. Light is the paint which colours bodies. You know that ordinary white light is made up of a series of beautiful colours (the spectrum), which I show you here. If I take all these spectrum or rainbow colours which are painted on this gla.s.s I can, as you see, recompose them into white light by rotating the disc with sufficient rapidity that they may get mixed together on the little screen at the back of your eye. White light then is a mixture of a number of colours.

Just ask yourselves this question. Why is this piece of ribbon white?

The white light falls upon it. White light is made up of all those colours you saw just now upon the screen. The light is reflected from this ribbon exactly as it fell upon the ribbon. The whole of those colours come off together, and that ribbon is white because the whole of the colours of the spectrum are reflected at the same moment. Why is that ribbon green? The white light falls upon the ribbon--the violet, the indigo, the red, the blue, the orange, and the yellow, are absorbed by the dye of the ribbon, and you do not see them. The ribbon, as it were, drinks in all these colours, but it cannot drink in the green. And reflecting the green of the spectrum, you see that ribbon green because the ribbon is incapable of absorbing the green of the white light. Why is this ribbon red? For the same reason. It can absorb the green which the previous piece of ribbon could not absorb, but it cannot absorb the red. The fact is, colour is not an inherent property of a body. If you ask me why that ribbon is green, and why this ribbon is red, the real answer is, that the red ribbon has absorbed every colour except the red, and the green ribbon every colour except the green, not because they are of themselves red and green but because they have the power of reflecting those colours from their surfaces.

This then is the consummated work of our tinder-box. Our tinder-box set fire to the match, and the match set fire to the candle, whilst the heat and the light of the candle are the finished work of the candle that the tinder-box lighted.

The clock warns me that I must bring to an end my story of a tinder-box.

To be sure, the tinder-box is a thing of the past, but I hope its story has not been altogether without teaching. Let me a.s.sure you that the failure, if failure there be, is not the fault of the story, but of the story-teller. If some day, my young friends, you desire to be great philosophers--and such desire is a high and holy ambition--be content in the first instance to listen to the familiar stories told you by the commonest of common things. There is nothing, depend upon it, too little to learn from. In time you will rise to higher efforts of thought and intellectual activity, but you will be primed for those efforts by the grasp you have secured in your studies of every-day phenomena.

"Great things are made of little things, And little things go lessening, till at last Comes G.o.d behind them."

THE END.

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