The Chemical History of a Candle - Michael Faraday (best free ebook reader for android .txt) 📗

or 90, or 85 of lead (which he calls weak platinum), and he then places it in the kind of vessel that you see before you. Suppose we had the mixture here; we should have to make it hot, and then throw in air upon the surface. The combustible metal—that is, the lead—and the part that will oxidise, are thoroughly oxidised; the litharge would flow out in a fused state into a vessel placed to receive it, and the platinum remains behind.

[Illustration: Fig. 36.]

Here is the process which Deville adopts for the purpose of casting off the lead, after he has got out the platinum from the ore. (Having made use of your friend, you get rid of him as quickly as you can.) He gets his heat by applying the combination of oxygen and hydrogen, or of carburetted fuel, for the purpose of producing a fire. I have here a source of coal-gas; there I have a source of hydrogen; and here I have a source of oxygen. I have here also one of the blowpipes used by Deville in his process for working platinum in the way I have spoken of. There are two pipes, and one of them goes to the source of coal-gas, and the other to the supply of oxygen.

[Illustration: Fig. 37.]

By uniting these we obtain a flame of such a heat as to melt platinum. You will, perhaps, hardly imagine what the heat is, unless you have some proof of it; but you will soon see that I have actually the power of melting platinum. Here is a piece of platinum-foil running like wax under the flame which I am bringing to bear against it. The question, however, is whether we shall get heat enough to melt, not this small quantity, but large masses—many pounds of the metal. Having obtained heat like this, the next consideration is what vessel is he to employ which could retain the platinum when so heated, or bear the effects of the flame? Such vessels are happily well supplied at Paris, and are formed of a substance which surrounds Paris; it is a kind of chalk (called, I believe, by geologists, calcaire grossière), and it has the property of enduring an extreme degree of heat. I am now going to get the highest heat that we can obtain. First, I shew you the combustion of hydrogen by itself. I have not a large supply, because the coal-gas is sufficient for most of our purposes. If I put a piece of lime obtained from this chalk into the gas, you see we get a pretty hot flame, which would burn one's fingers a good deal But now let me subject a piece of it to the joint action of oxygen and hydrogen. I do this for the purpose of shewing you the value of lime as a material for the furnaces and chambers that are to contain the substances to be operated on, and that are consequently to sustain the action of this extreme heat. Here we have the hydrogen and the oxygen, which will give the most intense heat that can be obtained by chemical action; and if I put a piece of lime into the flame, we get what is called the lime-light. Now, with all the beauty and intensity of action which you perceive, there is no sensible deterioration of the lime except by the mechanical force of the current of gases rushing from the jet against the lime, sweeping away such particles as are not strongly aggregated. "Vapour of lime" some call it; and it may be so, but there is no other change of the lime than that under the action of heat of this highly-exalted chemical condition, though almost any other substance would melt at once.

Then, as to the way in which the heat is applied to the substance. It is all very well for me to take a piece of antimony, and fuse it in the flame of a blowpipe. But if I tried this piece in the ordinary lamp flame, I should do nothing; if I tried a smaller piece, I should do little or nothing; and if I tried a still smaller piece, I should do little or nothing; yet I have a condition which will represent what Deville carries to the highest possible extent, and which we all carry to the highest extent, in the use of the blowpipe. Suppose I take this piece of antimony: I shall not be able to melt it in that flame of the candle by merely holding it there; yet, by taking pains, we can even melt a piece of platinum there. This is a preparation which I made for the purpose of proving the fusibility of platinum in a common candle. There is a piece of wire, drawn by that ingenious process of Dr. Wollaston's, not more than the three-thousandth part of an inch in diameter. He put the wire into the middle of a cylinder of silver, and drew both together until the whole compound was exceedingly thin; and then he dissolved away the silver by nitric acid. There was left in the centre a substance which I can scarcely see with an eye-glass, but which I know is there, and which I can make visible, as you see, by putting it into the candle, where the heat makes it glow like a spark. I have again and again tried this experiment up-stairs in my own room, and have easily fused this platinum-wire by a common candle. You see we have, therefore, heat enough in the candle, as in the voltaic battery, or in the highly-exalted combustion of the blowpipe, but we do not supply a continuous source of heat. In the very act of this becoming ignited, the heat radiates so fast that you cannot accumulate enough to cause the fusion of the wire, except under the most careful arrangement. Thus I cannot melt that piece of antimony by simply putting it into the candle; but if I put it upon charcoal, and drive the fiery current against it, there will be heat enough to melt it. The beauty of the blowpipe is, that it sends hot air (making hot air by the combustion of the flame) against the thing to be heated. I have only to hold the antimony in the course of that current, and particle by particle of the current impinges upon the antimony, and so we get it melted. You now see it red-hot, and I have no doubt it will continue to burn if I withdraw it from the flame and continue to force the air on it. Now, you see it burning without any heat but that of its own combustion, which I am keeping up by sending the air against it. It would go out in a moment if I took away the current of air from it; but there it is burning, and the more air I give it, by this or any other action, the better it is. So, then, we have here not merely a mighty source of heat, but a means of driving the heat forcibly against substances.

Let me shew you another experiment with a piece of iron. It will serve two purposes—shewing you what the blowpipe does as a source of heat, and what it does by sending that heat where it is wanted. I have taken iron in contrast with silver or other metals, that you may see the difference of action, and so be more interested in the experiment. Here is our fuel, the coal-gas; and here our oxygen. Having thus my power of heat, I apply it to the iron, which, as you see, soon gets red-hot. It is now flowing about like a globule of melted mercury. But observe, I cannot raise any vapour: it is now covered with a coat of melted oxide, and unless I have a great power in my blowpipe, it is hardly possible to break through it. Now, then, you see these beautiful sparks: you have not only a beautiful kind of combustion, but you see the iron is being driven off, not producing smoke, but burning in a fixed condition. How different this is from the action of some other metals—that piece of antimony, for instance, which we saw just now throwing off abundance of fumes. We can, of course, burn away this iron by giving plenty of air to it; but with the bodies which Deville wants to expose to this intense heat he has not that means: the gas itself must have power enough to drive off the slag which forms on the surface of the metal, and power to impinge upon the platinum so as to get the full contact that he wants for the fusion to take place. We see here, then, the means to which he resorts—oxygen, and either coal-gas or water-gas[19], or pure hydrogen, for producing heat, and the blowpipe for the purpose of impelling the heated current upon the metals.

I have two or three rough drawings here, representing the kind of furnaces which he employs. They are larger, however, than the actual furnaces he uses. Even the furnace in which he carries on that most serious operation of fusing fifty pounds of platinum at once is not much more than half the size of the drawing. It is made of a piece of lime below and a piece of lime above. You see how beautifully lime sustains heat without altering in shape; and you may have thought how beautifully it prevents the dissipation of the heat by its very bad conducting powers.

[Illustration: Fig. 38]

While the front part of the lime which you saw here was so highly ignited, I could at any moment touch the back of it without feeling any annoyance from the heat So, by having a chamber of lime of this sort, he is able to get a vessel to contain these metals with scarcely any loss of heat. He puts the blowpipes through these apertures, and sends down these gases upon the metals, which are gradually melted. He then puts in more metal through a hole at the top. The results of the combustion issue out of the aperture which you see represented. If there be strips of platinum, he pushes them through the mouth out of which the heated current is coming, and there they get red-hot and white-hot before they get into the bath of platinum. So he is able to fuse a large body of platinum in this manner. When the platinum is melted, he takes off the top and pours out from the bottom piece, like a crucible, and makes his cast. This is the furnace by which he fuses his forty pounds or fifty pounds of platinum at once. The metal is raised to a heat that no eye can bear. There is no light and shadow, no chiaro-oscuro there; all is the same intensity of glow. You look in, and you cannot see where the metal or the lime is; it is all as one. We have, therefore, a platform with a handle, which turns upon an axis, that coincides with the gutter that is formed for the pouring of the metal; and when all is known to be ready, by means of dark glasses, the workmen take off the top piece and lift up the handle, and the mould being then placed in a proper position, he knows that the issue of the metal will be exactly in the line of the axis. No injury has ever happened from the use of this plan. You know with what care it is necessary to carry such a vessel of mercury as we have here, for fear of turning it over on one side or the other; but if it be a vessel of melted platinum, the very greatest care must be used, because the substance is twice as heavy: yet no injury has been done to any of the workmen in this operation.

I have said that Deville depends upon intense heat for carrying off vapour; and this brings me to the point of shewing how vapours are carried off. Here is a basin of mercury, which boils easily, as you know, and gives us the opportunity of observing the facts and principles which are to guide us. I have here two poles of the battery, and if I bring them into contact with the mercury, see what a development of vapour we have. The mercury is flying off rapidly; and I might, if I pleased, put all the company around me in a bath of mercury vapour. And so, if we take this piece of lead and treat it in the same way, it will also give off vapour. Observe the