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The History of Chemistry. Thomas Thomson
Читать онлайн.Название The History of Chemistry
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isbn 4064066382971
Автор произведения Thomas Thomson
Жанр Математика
Издательство Bookwire
Tin, in the time of Pliny, was used for covering the inside of copper vessels, as it is at this day. And, no doubt, the process still followed is of the same nature as the process used by the ancients for tinning copper. Pliny remarks, with surprise, that copper thus tinned does not increase in weight. Now Bayen ascertained that a copper pan, nine inches in diameter, and three inches three lines in depth, when tinned, only acquired an additional weight of twenty-one grains. These measures and weights are French. When we convert them into English, we have a copper pan 9·59 inches in diameter, and 3·46 inches deep, which, when tinned, increased in weight 17·23 troy grains. Now the surface of the copper pan, thus tinned, was 176·468 square inches. Hence it follows, that a square inch of copper, when tinned, increases in weight only 0·097 grains. This increase is so small, that we may excuse Pliny, who probably had never seen the increase of weight determined, except by means of a rude Roman statera, for concluding that there was no increase of weight whatever.
Tin was employed by the ancients for mirrors: but mirrors of silver were gradually substituted; and these in Pliny’s time had become so common, that they were even employed by female servants or slaves.
That Pliny’s knowledge of the properties of tin was very limited, and far from accurate, is obvious from his assertion that tin is less fusible than silver.58 It is true that the ancients had no measure to determine the different degrees of heat; but as tin melts at a heat under redness, while silver requires a bright red heat to bring it into fusion, a single comparative trial would have shown him which was most fusible. This trial, it is obvious, had never been made by him.
The ancients seem to have been ignorant of the method of tinning iron. At least, no reference to tin plate is made by Pliny, or by any other ancient author, that I have had an opportunity of consulting.
It would appear from Pliny, that both copper and brass were tinned by the Gauls at an early period. Tinned brass was called æra coctilia, and was so beautiful that it almost passed for silver. Plating (or covering the metal with plates of silver), was gradually substituted for tinning; and finally gilding took the place of plating. The trappings of horses, chariots, &c., were thus ornamented. Pliny nowhere gives a description of the process of plating; but there can be little doubt that it was similar to that at present practised. Gilding was accomplished by laying an amalgam of gold on the copper or brass, as at present.
7. Lead appears also to have been in common use among the Egyptians, at the time of Moses.59 It was distinguished among the Romans by the name of plumbum nigrum. In Pliny’s time the lead-mines existed chiefly in Spain and Britain. In Britain lead was so abundant, that it was prohibited to extract above a certain quantity in a year. The mines lay on the surface of the earth. Derbyshire was the county in which lead ores were chiefly wrought by the Romans. The rich mines in the north of England seem to have been unknown to them.
Pliny was of opinion that if a lead-mine, after being exhausted, be shut up for some time, the ore will be again renewed.
In the time of Pliny leaden pipes were commonly used for conveying water. The vulgar notion that the ancients did not know that water will always rise in pipes as high as the source from which it proceeds, and that it was this ignorance which led to the formation of aqueducts, is quite unfounded. Nobody can read Pliny without seeing that this important fact was well known in his time.
Sheet lead was also used in the time of Pliny, and applied to the same purposes as at present. But lead was much higher priced among the ancients than it is at present. Pliny informs us that its price was to that of tin as 7 to 10. Hence it must have sold at the rate of 6s. 0¼d. per pound. The present price of lead does not much exceed three halfpence the pound. It is therefore only 1–48th part of the price which it bore in the time of Pliny. This difference must be chiefly owing to the improvements made by the moderns in working the mines and smelting the ores of lead.
Tin, in Pliny’s time, was used as a solder for lead. For this purpose it is well adapted, as it is so much easier smelted than lead. But when he says that lead is used also as a solder for tin, his meaning is not so clear. Probably he means an alloy of lead and tin, which, fusing at a lower point than tin, may be used to solder that metal. The addition of some bismuth reduces the fusing point materially; but that metal was unknown to the ancients.
Argentarium is an alloy of equal parts of lead and tin. Tertiarium, of two parts lead and one part tin. It was used as a solder.
Some preparations of lead were used by the ancients in medicine, as we know from the description of them given us by Dioscorides and Pliny. These preparations consisted chiefly of protoxide of lead and lead reduced to powder, and partially oxidized by triturating it with water in a mortar. They were applied to ulcers, and employed externally as astringents.
Molybdena was also employed in medicine. Pliny says it was the same as galena. From his description it is obvious that it was litharge; for it was in scales, and was more valued the nearer its colour approached to that of gold. It was employed, as it still is, for making plasters. Pliny gives us the process for making the plaster employed by the Roman surgeons. It was made by heating together 3 lbs. molybdena or litharge, 1 lb. wax, 3 heminæ, or 1½ pint, of olive oil. This process is very nearly the same as the one at present followed by apothecaries for making adhesive plaster.
Psimmythium, or cerussa, was the same as our white lead. It was made by exposing lead in sheets to the fumes of vinegar. It would seem probable from Pliny’s account, though it is confused and inaccurate, that the ancients were in the habit of dissolving cerussa in vinegar, and thus making an impure acetate of lead.
Cerussa was used in medicine. It constituted also a common white paint. At one time, Pliny says, it was found native; but in his time all that was used was prepared artificially.
Cerussa usta seems to have been nearly the same as our red lead. It was formed accidentally from cerussa during the burning of the Pyræus. The colour was purple. It was imitated at Rome by burning silis marmarosus, which was probably a variety of some of our ochres.
8. Besides the metals above enumerated, the ancients were also acquainted with quicksilver. Nothing is known about the first discovery of this metal; though it obviously precedes the commencement of history. I am not aware that the term occurs in the writings of Moses. We have therefore no evidence that it was known to the Egyptians at that early period; nor do I find any allusion to it in the works of Herodotus. But this is not surprising, as that author confines himself chiefly to subjects connected with history. Dioscorides and Pliny both mention it as common in their time. Dioscorides gives a method of obtaining it by sublimation from cinnabar. It is remarkable, because it constitutes the first example of a process which ultimately led to distillation.60
Cinnabar is also described by Theophrastus. The term minium was applied to it also, till in consequence of the adulteration of cinnabar with red lead, the term minium came at last to be restricted to that preparation of lead. Theophrastus describes an artificial cinnabar, which came from the country above Ephesus. It was a shining red-coloured sand, which was collected and reduced to a fine powder by pounding it in vessels of stone. We do not know what it was. The native cinnabar was found in Spain, and was used chiefly as a paint. Dioscorides employs minium as the name for what we at present call cinnabar, or bisulphuret of mercury. His cinnabar was a red paint from Africa, produced in such small quantity that painters could scarcely procure enough of it to answer their purposes.
Mercury is described by Pliny as existing native in the mines of Spain, and Dioscorides gives the process for extracting it from cinnabar. It was employed in gilding precisely as it is by the moderns. Pliny was aware of its great specific gravity, and of the readiness with which it dissolves gold. The amalgam was squeezed through leather, which separated most of the quicksilver. When the solid amalgam remaining was heated, the mercury was driven off and pure gold remained.
It is obvious from what Dioscorides says, that the properties