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

iron, but he has a fine philosophy there, or practises it, when he gets his welding. I can shew you here this beautiful circumstance of the sticking together of the particles up to the fullest possible intensity of their combination. If you were to go into the workshops of Mr. Matthey, and see them hammering and welding away, you would see the value of the experiment I am about to shew you. I have here some platinum-wire. This is a metal which resists the action of acids, resists oxidation by heat, and change of any sort; and which, therefore, I may heat in the atmosphere without any flux. I bend the wire so as to make the ends cross: these I make hot by means of the blowpipe, and then, by giving them a tap with a hammer, I shall make them into one piece. Now that the pieces are united, I shall have great difficulty in pulling them apart, though they are joined only at the point where the two cylindrical surfaces came together. And now I have succeeded in pulling the wire apart, the division is not at the point of welding, but where the force of the pincers has cut it, so that the junction we have effected is a complete one. This, then, is the principle of the manufacture and production of platinum in the old way.

The treatment which Deville proposes to carry out, and which he has carried out to a rather large extent in reference to the Russian supply of platinum, is one altogether by heat, having little or no reference to the use of acids. That you may know what the problem is, look at this table, which gives you the composition of such a piece of platinum ore as I shewed you just now. Wherever it comes from, the composition is as complicated, though the proportions vary:—

Platinum, . . . . . 76.4
Iridium,. . . . . . 4.3
Rhodium,. . . . . . 0.3
Palladium,. . . . . 1.4
Gold, . . . . . . . 0.4
Copper, . . . . . . 4.1
Iron, . . . . . . . 11.7
Osmide of Iridium,. 0.5
Sand, . . . . . . . 1.4
                   ——-
                   100.5

This refers to the Uralian ore. In that state of combination, as shewn in the table, the iridium and osmium are found combined in crystals, sometimes to the amount of 0.5 per cent., and sometimes 3 or 4 per cent. Now, this Deville proposes to deal with in the dry way, in the place of dealing with it by any acid.

I have here another kind of platinum; and I shew it to you for this reason. The Russian Government, having large stores of platinum in their dominions, have obtained it in a metallic state, and worked it into coin. The coin I have in my hand is a twelve silver rouble piece. The rouble is worth three shillings, and this coin is, therefore, of the value of thirty-six shillings. The smaller coin is worth half that sum; and the other, half of that. The metal, however, is unfit for coinage. When you have the two metals, gold and silver, used for coinage, you have a little confusion in the value of the two in the market; but when you have three precious metals (for you may call platinum a precious metal) worked into coin, they will be sure to run counter to one another. Indeed, the case did happen, that the price of platinum coin fixed by the Government was such, that it was worth while to purchase platinum in other countries, and make coin of it, and then take it into that country and circulate it. The result was, that the Russian Government stopped the issue. The composition of this coin is—platinum, 97.0; iridium, 1.2; rhodium, 0.5; palladium, 0.25; a little copper, and a little iron. It is, in fact, bad platinum: it scales, and it has an unfitness for commercial use and in the laboratory, which the other well-purified platinum has not. It wants working over again.

Now, Deville's process depends upon three points,—upon intense heat, blowpipe action, and the volatility of certain metals. We know that there are plenty of metals that are volatile; but this, I think, is the first time that it has been proposed to use the volatility of certain metals—such as gold and palladium—for the purpose of driving them off and leaving something else behind. He counts largely upon the volatility of metals which we have not been in the habit of considering volatile, but which we have rather looked upon as fixed; and I must endeavour to illustrate these three points by a few experiments. Perhaps I can best show you what is required in the process of heating platinum by using that source of heat which we have here, and which seems to be almost illimitable—namely, the voltaic battery; for it is only in consequence of the heat that the voltaic battery affects the platinum. By applying the two extremities of the battery to this piece of platinum-wire, you will see what result we shall obtain. You perceive that we can take about this heating agent wherever we like, and deal with it as we please, limiting it in any way. I am obliged to deal carefully with it; but even that circumstance will have an interest for you in watching the experiment. Contact is now made. The electric current, when compressed into thin conducting-wires offering resistance, evolves heat to a large extent; and this is the power by which we work. You see the intense glow immediately imparted to the wire; and if I applied the heat continuously, the effect of the current would be to melt the wire. As soon as the contact is broken, the wire resumes its former appearance; and now that we make contact again, you perceive the glow as before. [The experiment was repeated several times in rapid succession.] You can see a line of light, though you can scarcely perceive the wire; and now that it has melted with the great heat, if you examine it, you will perceive that it is indeed a set of irregularities from end to end—a set of little spheres, which are strung upon an axis of platinum running through it. It is that wire which Mr. Grove described as being produced at the moment when fusion of the whole mass is commencing. In the same manner, if I take a tolerably thick piece of platinum, and subject it to the heat that can be produced by this battery, you will see the brilliancy of the effect produced. I shall put on a pair of spectacles for the experiment, as there is an injurious effect of the voltaic spark upon the eyes, if the action is continued; and it is neither policy nor bravery to subject any organ to unnecessary danger; and I want, at all events, to keep the full use of my eyes to the end of the lecture.

You now see the action of the heat upon the piece of platinum—heat so great as to break in pieces the plate on which the drops of metal fall. You perceive, then, that we have sufficiently powerful sources of heat in nature to deal with platinum. I have here an apparatus by which the same thing can be shewn. Here is a piece of platinum, which is put into a crucible of carbon made at the end of one pole of the battery, and you will see the brilliant light that will be produced. There is our furnace, and the platinum is rapidly getting heated; and now you perceive that it is melted, and throwing off little particles. What a magnificent philosophical instrument this is. When you look at the result, which is lying upon the charcoal, you will see a beautifully fused piece of platinum. It is now a fiery globule, with a surface so bright, and smooth, and reflecting, that I cannot tell whether it is transparent, or opaque, or what. This, then, will give you an idea of what has to be done by any process that pretends to deal with thirty, or forty, or fifty pounds of platinum at once.

Let me now tell you briefly what Deville proposes to do. First of all, he takes this ore, with its impurities, and mixes it (as he finds it essential and best) with its own weight of sulphuret of lead—lead combined with sulphur. Both the lead and the sulphur are wanted; for the iron that is there present, as you see by the table, is one of the most annoying substances in the treatment that you can imagine, because it is not volatile; and while the iron remains adhering to the platinum, the platinum will not flow readily. It cannot be sent away by a high temperature—sent into the atmosphere so as to leave the platinum behind. Well, then, a hundred parts of ore and a hundred parts of sulphuret of lead, with about fifty parts of metallic lead, being all mingled together in a crucible, the sulphur of the sulphuret takes the iron, the copper, and some of the other metals and impurities, and combines with them to form a slag; and as it goes on boiling and oxidising, it carries off the iron, and so a great cleansing takes place.

Now, you ought to know that these metals, such as platinum, iridium, and palladium, have a strong affinity for such metals as lead and tin, and upon this a great deal depends. Very much depends upon the platinum throwing out its impurities of iron and so forth, by being taken up with the lead present in it. That you may have a notion of the great power that platinum has of combining with other metals, I will refer you to a little of the chemist's experience—his bad experience. He knows very well that if he takes a piece of platinum-foil, and heats a piece of lead upon it, or if he takes a piece of platinum-foil, such as we have here, and heats things upon it that have lead in them, his platinum is destroyed. I have here a piece of platinum, and if I apply the heat of the spirit-lamp to it, in consequence of the presence of this little piece of lead which I will place on it, I shall make a hole in the metal. The heat of the lamp itself would do no harm to the platinum, nor would other chemical means; but because there is a little lead present, and there is an affinity between the two substances, the bodies fuse together at once. You see the hole I have made. It is large enough to put your finger in, though the platinum itself was, as you saw, almost infusible, except by the voltaic battery. For the purpose of shewing this fact in a more striking manner, I have taken pieces of platinum-foil, tin-foil, and lead-foil, and rolled them together; and if I apply the blowpipe to them, you will have, in fact, a repetition on a larger scale of the experiment you saw just now when the lead and platinum came together, and one spoiled the other. When the metals are laid one upon the other, and folded together and heat applied, you will not only see that the platinum runs to waste, but that at the time when the platinum and lead are combined there is ignition produced—there is a power of sustaining combustion. I have taken a large piece, that you may see the phenomenon on a large scale. You saw the ignition and the explosion which followed, of which we have here the results—the consequence of the chemical affinity between the platinum and the metals combined with it, which is the thing upon which Deville founds his first result.

When he has melted these substances and stirred them well up, and so obtained a complete mixture, he throws in air upon the surface to burn off all the sulphur from the remaining sulphuret of lead; and at last he gets an ingot of lead with platinum—much lead, comparatively, and little platinum. He gets that in the crucible with a lot of scoriæ and other things, which he treats afterwards. It is that platiniferous lead which we have to deal with in our future process. Now, let me tell you what he does with it. His first object is to get rid of the lead. He has thrown out all the iron, and a number of other things, and he has got this kind of compound indicated in the table. He may get it as high as 78 per cent. of platinum, and 22 of lead; or 5, or 10, or 15 of platinum, and 95,