SECTION 16.CARBURETS.
On the supposition that metals combine with charcoal, the appropriate names for the compounds would becarburetsof the respective metals. This combination, if it exist at all, seems very rare, that with iron being the only one generally acknowledged. No combinations of carbone with the earths and alkalies, have, as far as I know, been noticed; and those with the elements oxygen, hydrogen, sulphur and phosphorus have been described in the former volume. Since that was printed an ingenious experimental essay on the “Sulphuret of carbonor alcohol of sulphur,” has been published by Berzelius and Dr. Marcet. Some account of this compound, under the namecarburetted sulphur, has been given (Vol. 1.page 462); but the additional information is of sufficient importance to require notice here. The pure liquid is of sp. gr. 1.272; and the elasticity of its vapour at 66° is equal to 10.76 inches. It burns with a blue flameand sulphureous odour, without sensibly depositing water on cold glass exposed to the fumes. It has an acrid, pungent taste, and a nauseous smell, differing from sulphuretted hydrogen. By various experiments it was found to consist of sulphur and carbone in the ratio of 85 to 15 nearly; that is, 2 atoms of sulphur to 1 of carbone. From other experiments it did not appear to contain any hydrogen.
From some experiments I made in June 1818, on the combustion of the vapour of carburet of sulphur in oxygen gas, I was led to suspect at least, that an atom of hydrogen attaches to the two of sulphur and one of carbone in its constitution. But not having an opportunity to pursue the subject, I merely make the observation for future experience to determine upon the question.
There are three distinct substances which are now commonly believed to be constituted of carbone and iron, known by the names ofPlumbago, or black lead,Cast IronandSteel.
Plumbago is a natural production, found in greatest perfection in theBorrowdale mine, near Keswick, Cumberland. It is chiefly used in making pencils.—It seems to be constituted of carbone and iron by the concurrent experience of all who have examined it: but the proportions are not uniform, some having found 10 and others only 5 per cent. of iron in it. From this circumstance it would seem doubtful whether iron is an essential element. As carbone is known to be exhibited in various forms of aggregation, it is not improbable that plumbago may be one of those forms; it is evidently not a mere mixture of common charcoal and iron, or its oxide.
Cast ironor crude iron is the metal when first extracted from the ore; it usually contains carbone, oxygen, phosphorus and silica, in small proportions, with perhaps other earths occasionally. It cannot be considered as having these elements united in definite proportions; for they vary much, and probably give to crude iron its several modifications. Cast iron contains about 80 per cent. of its weight of iron in a state capable of solution in dilute sulphuric acid, and yielding a corresponding quantity of hydrogen gas. The residue, in a specimen I examined, was nearly as magnetic as iron itself. When treated with boiling muriatic acid, the insoluble part was reduced to2½ per cent. upon the original weight of the iron, and some hydrogen gas given out. It was then about as magnetic as the common black oxide of iron; when heated it assumed a glowing red and lost nearly ½ grain; it was still magnetic, and boiling muriatic acid extracted more iron from it.
The hydrogen gas from dilute sulphuric acid and cast iron contains no carbonic acid in my experience; neither does it yield any when exploded with pure oxygen gas.
The small residuum after treating cast iron with acids was found by Bergman and others to resemble plumbago, being constituted chiefly of carbone and iron.
From the above it would seem that cast iron consists chiefly of pure iron, with the addition of very small proportions of oxygen and carbone; the oxygen may be about 1 per cent. and the carbone about 2. These proportions, though sufficient to modify the properties of iron to a certain extent, can scarcely be considered as constituting cast iron a homogeneous mass.
Steel.This most important modification of iron has engaged the attention of many chemists and metallurgists. It may be procured, but not equally pure, by different methods. One is to keep the cast ironfor a considerable time in fusion and in a very high degree of heat; whilst its surface is covered with melted scoriæ, so as to preclude the contact of the atmosphere with the iron. This, it is conceived, gives time for the carbone and oxygen to combine and escape in the form of carbonic acid. This steel is of inferior purity.
Steel of cementation is made by stratifying bars of pure iron with charcoal powder in large earthen crucibles, carefully closed up with clay. These are exposed to a high degree of heat in a furnace for 8 or 10 days. This is calledblisteredsteel, from the appearance of blisters on its surface.
Cast steel is made from blistered steel by breaking the bars and putting them into a large crucible with pounded glass and charcoal. The crucible is closed with a lid of the same ware and placed in an air furnace. When the fusion is complete the metal is cast into ingots. This is the most valuable and probably the purest steel.
When steel is heated red and plunged into cold water, it ishardened; that is, it becomes much harder than iron or than steel without this operation. Hardened steel is brittle, and cannot be extended by the hammer or corroded by a file till it is again softenedby being heated and then gradually cooled.
One of the most remarkable properties of hardened steel is that of beingtempered, as it is called; by which it is adapted to the different purposes of the manufacturing artists. Tempering consists in heating the hardened steel till it acquires a straw colour for edge tools, a blue colour for watch springs, and elastic articles in general; &c. &c.
Hardened steel is qualified to acquire magnetism, and to retain it so as to become a permanent magnet. This power of retaining magnetism distinguishes steel from pure iron.
From the above account of steel, it is evident there is an essential difference between it and pure iron. That difference consists, according to the common opinion, in steel being acarburetof iron, or carbone and iron united. The fact of the union of carbone and iron in the formation of steel does not seem to me satisfactorily proved. Mr. Collier asserts that iron gains about ¹/₁₈₀th of its weight by being converted into steel.[18]But Mr. Mushet found that though steel gains weight upon the iron whencopiously imbedded in charcoal, yet it loses weight if the charcoal is only ¹/₉₀ or ¹/₁₀₀ of the weight of the iron.[19]The same ingenious gentleman seems to estimate the carbone in cast steel, from synthetic experiments, to be ¹/₁₀₀th of its weight.
From analytic experiments, however, there does not appear reason to believe that steel contains so much, if any charcoal. Pure steel dissolved in dilute sulphuric acid gives hydrogen gas containing no carbonic acid nor oxide, neither is there any appreciable residuum of any kind in general.
On considering all the circumstances, I am inclined to believe, that the properties which distinguish steel from iron are rather owing to a peculiar crystallization or arrangement of the ultimate particles of iron, than to their combination with carbone or any other substance. In all cases where steel is formed, the mass is brought into a liquid form, or nearly approaching to it, a circumstance which allows the particles to be subject to the law of crystallization. We see that great change is made in steel by the meretemperingof it, which cannot be ascribed to the loss or gain of any substance, but to somemodification of the internal arrangement of its particles. Why then may not its differences from iron be ascribed to the same cause? It is allowed that steel, by being repeatedly heated and hammered, becomes iron: that is, it should seem, the change of figure disturbs the regular arrangement of the particles. And it may be further observed, in corroboration of the opinion that cast iron is capable of being made permanently magnetic, from its having been in fusion more probably than from its near approximation to steel in its component parts. The most powerful artificial magnets, after being forged of steel, are said to undergo the operation of steelifying again, before they are hardened finally to receive the magnetic virtue.