The Chemical History of a Candle

[Delivered before the ROYAL INSTITUTION, on Friday, February 22, 1861.] Whether I was to have the honour of appearing before you this evening or not, seemed to be doubtful upon one or two points. One of these I will mention immediately; the other may or may not appear during the course of the hour that follows. The first point is this. When I was tempted to promise this subject for your attention this evening, it was founded upon a promise, and a full intent of performing that promise, on the part of my friend Deville, of Paris, to come here to shew before you a phenomenon in metallurgic chemistry not common. In that I have been disappointed. His intention was to have fused here some thirty or forty pounds of platinum, and so to have made manifest, through my mouth and my statement, the principles of a new process in metallurgy, in relation to this beautiful, magnificent, and valuable metal; but circumstances over which neither he nor I, nor others concerned, have sufficient control, have prevented the fulfilment of that intention; and the period at which I learned the fact was so recent, that I could hardly leave my place here to be filled by another, or permit you, who in your kindness have come to hear what might be said, to remain unreceived in the best manner possible to me under the circumstances. I therefore propose to state, as well as I can, what the principles are on which M. Deville proceeds, by means of drawings, and some subordinate or inferior experiments. The metal platinum, of which you see some very fine specimens on the table, has been known to us about a hundred years. It has been wrought in a beautiful way in this country, in France, and elsewhere, and supplied to the consumer in ingots of this kind, or in plates, such as we have here, or in masses, that by their very fall upon the table indicate the great weight of the substance, which is, indeed, nearly at the head of all substances in that respect. This substance has been given to us hitherto mainly through the philosophy of Dr. Wollaston, whom many of us know, and it is obtained in great purity and beauty. It is a very remarkable metal in many points, besides its known special uses. It usually comes to us in grains. Here is a very fine specimen of native platinum in grains. Here is also a nugget or ingot, and here are some small pieces gathered out of certain alluvial soils in Brazil, Mexico, California, and the Uralian districts of Russia. It is strange that this metal is almost always found associated with some four or five other metals, most curious in their qualities and characteristics. They are called platiniferous metals; and they have not only the relation of being always found associated in this manner, but they have other relations of a curious nature, which I shall point out to you by a reference to one of the tables behind me. This substance is always native—it is always in the metallic state; and the metals with which it is found connected, and which are rarely found elsewhere, are palladium, rhodium, iridium, osmium, and ruthenium. We have the names in one of the tables arranged in two columns, representing, as you see, two groups—platinum, iridium, and osmium constituting one group; and ruthenium, rhodium, and palladium the other. Three of these have the chemical equivalent of 98½, and the others a chemical equivalent of about half that number. Then the metals of one group have an extreme specific gravity—platinum being, in fact, the lightest of the three, or as light as the lightest. Osmium has a specific gravity of 21.4, and is the heaviest body in nature; platinum is 21.15, and iridium the same; the specific gravity of the other three being only about half that, namely, 11.3, 12.1, and 11.8. Then there is this curious relation, that palladium and iridium are very much alike, so that you would scarcely know one from the other, though one has only half the weight of the other, and only half the equivalent power. So with iridium and rhodium, and osmium and ruthenium, which are so closely allied that they make pairs, being separated each from its own group. Then these metals are the most infusible that we possess. Osmium is the most difficult to fuse: indeed, I believe it never has been fused, while every other metal has. Ruthenium comes next, iridium next, rhodium next, platinum next (so that it ranks here as a pretty fusible metal, and yet we have been long accustomed to speak of the infusibility of platinum), and next comes palladium, which is the most fusible metal of the whole. It is a curious thing to see this fine association of physical properties coming out in metals which are grouped together somehow or other in nature, but, no doubt, by causes which are related to analogous properties in their situation on the surface of the earth, for it is in alluvial soils that these things are found. Now, with regard to this substance, let me tell you briefly how we get it. The process used to be this. The ore which I shewed you just now was taken, and digested in nitro-muriatic acid of a certain strength, and partly converted into a solution, with the leaving behind of certain bodies that I have upon the table. The platinum being dissolved with care in acids, to the solution the muriate of ammonia was added, as I am about to add it here. A yellow precipitate was then thrown down, as you perceive is the case now; and this, carefully washed and cleansed, gave us that body [pointing to a specimen of the chloride of platinum and ammonium], the other elements, or nearly all, being ejected. This substance being heated, gave us what we call platinum sponge, or platinum in the metallic state, so finely divided as to form a kind of heavy mass or sponge, which, at the time that Dr. Wollaston first sent it forth, was not fusible for the market or in the manufacturers’ workshops, inasmuch as the temperature required was so high, and there were no furnaces that could bring the mass into a globule, and cause the parts to adhere together. Most of our metals that we obtain from nature, and work in our shops, are brought at last into a mass by fusion. I am not aware that there is in the arts or sciences any other than iron which is not so. Soft iron we do not bring together by fusion, but by a process which is analogous to the one that was followed in the case of platinum, namely, welding; for these divided grains of spongy platinum having been well washed and sunk in water for the purpose of excluding air, and pressed together, and heated, and hammered, and pressed again, until they come into a pretty close, dense, compact mass, did so cohere, that when the mass was put into the furnace of charcoal, and raised to a high temperature, the particles, at first infinitely divided—for they were chemically divided—adhered the one to the other, each to all the rest, until they made that kind of substance which you see here, which will bear rolling and expansion of every kind. No other process than that has hitherto been adopted for the purpose of obtaining this substance from the particles by solution, precipitation, ignition, and welding. It certainly is a very fine thing to see that we may so fully depend upon the properties of the various substances we have to deal with; that we can, by carrying out our processes, obtain a material like this, allowing of division and extension under a rolling mill—a material of the finest possible kind, the parts being held together, not with interstices, not with porosity, but so continuous that no fluids can pass between them; and, as Dr. Wollaston beautifully shewed, a globule of platinum fused by the voltaic battery and the oxyhydrogen blowpipe, when drawn into a wire, was not sounder or stronger than this wire made by the curious coalescence of the particles by the sticking power that they had at high temperatures. This is the process adopted by Messrs. Johnson and Matthey, to whose great kindness I am indebted for these ingots and for the valuable assistance I have received in the illustrations. The treatment, however, that I have to bring before you is of another kind; and it is in the hope that we shall be able before long to have such a thing as the manufacture of platinum of this kind, that I am encouraged to come before you, and tell you how far Deville has gone in the matter, and to give you illustrations of the principles on which he proceeds. I think it is but fair that you should see an experiment shewing you the way in which we get the adhesion of platinum. Probably you all know of the welding of iron: you go into the smith’s shop, and you see him put the handle of a poker on to the stem, and by a little management and the application of heat he makes them one. You have no doubt seen him put the iron into the fire and sprinkle a little sand upon it. He does not know the philosophy he calls into play when he sprinkles a little sand over the oxide of 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