[Fig. 251.] represents the construction of this ingenious apparatus, shown partly in section. a a is a vessel of iron, supported upon wooden standards b b, over the upper surface of which vessel a sheet or diaphragm, c c, of oiled cloth, or other suitable elastic material, is distended, and made fast at its edges by being bent over a flange, and packed or cemented to render the joints water-tight. A vertical pipe d is intended to conduct water to the interior of the vessel a, and, by a small elevation of the column, to create such upward pressure as shall give to the diaphragm a slight bulge like the swimming tub.
An endless web, e e e, passing over the surface of the diaphragm, is distended over three rollers, f g h, the lower of which, f, is in contact with the colour-roller i in the colour-trough K. On the axle of the roller i a pulley wheel is fixed, which allows the roller to be turned by a band from any first mover; or the roller may receive rotatory motion by a winch fixed on its axle. On this said axle there is also a toothed wheel, taking into another toothed wheel on the axle of the roller f; hence, the rotation of the colour-roller i in the one direction will cause the roller f to revolve in the opposite, and to carry forward the endless web e e e, over the elastic diaphragm, the web taking with it a stratum of colour received from the roller i, evenly distributed over its surface, and ready for the printer to dip his block into.
The axles of the rollers f and g turn in stationary bearings; but the axle of h is mounted in sliding nuts, which may be moved by turning the screws m, for the purpose of tightening the endless web. The axle of the colour-roller i turns in mortises, and may be raised by screws n, in order to bring its surface into contact with the endless web. To prevent too great a quantity of colour being taken up, the endless web passes through a long slit, or parallel aperture, in a frame o, which acts as a scraper or doctor, and is adjustable by a screw p, to regulate the quantity of colour carried up. The contents of the vessel a, and of the colour-trough K, may be discharged when required by a cock in the bottom of each. See [Paper Hangings], for the [Fondu] style.
CALOMEL. (Chlorure de Mercure, Fr.; Versüsstes Quecksilber, Germ.) The mild protochloride of mercury. The manufacture of this substance upon the great scale may be performed in two ways. The cheapest and most direct consists in mixing 11⁄8 part of pure quicksilver with 1 part of pure nitric acid, of sp. grav. from 1·2 to 1·25; and in digesting the mixture till no more metal can be dissolved, or till the liquid has assumed a yellow colour. At the same time, a solution of 1 part of common salt is made in 32 parts of distilled water, to which a little muriatic acid is added; and, when heated to nearly the boiling point, it is mixed with the mercurial solution. The two salts exchange bases, and a protochloride of mercury precipitates in a white powder, which, after being digested for some time in the acidulous supernatant liquor, is to be washed with the greatest care in boiling water. The circumstances which may injure the process are the following:—1. When less mercury is employed than the acid can dissolve, there is formed a deuto-nitrate of mercury, which forms some corrosive sublimate with the common salt, and causes a proportional defalcation of calomel. 2. If the liquors are perfectly neutral at the moment of mixing them, some subnitrate of mercury is thrown down, which cannot be removed by washing, and which gives a noxious contamination to the bland calomel. The acid prescribed in the above formula obviates this danger.
The second manner of manufacturing calomel is to grind very carefully 4 parts of corrosive sublimate (bi-chloride of mercury) with 3 parts of quicksilver, adding a little water or spirits to repress the noxious dust during the trituration. The mass is then introduced into a glass globe, and sublimed at a temperature gradually raised. The quicksilver combines with the deutochloride, and converts it into the protochloride, or calomel. The following formula, upon the same principle, was recommended to the chemical manufacturer in Brande’s Journal, for July, 1818:—
“Prepare an oxysulphate of mercury, by boiling 25 pounds of mercury with 35 pounds of sulphuric acid to dryness. Triturate 31 pounds of this dry salt with 20 pounds 4 ounces of mercury, until the globules disappear, and then add 17 pounds of common salt. The whole is to be thoroughly mixed, and sublimed in earthen vessels. Between 46 and 48 pounds of pure calomel are thus produced: it is to be washed and levigated in the usual way.” The above is the process used at Apothecaries’ Hall, London. The oxysulphate is made in an iron pot; and the sublimation is performed in earthen vessels. The crystalline crust or cake of calomel should be separated from the accompanying gray powder, which is nearest the glass, and consists of mercury mixed with corrosive sublimate.
An ingenious modification of the latter process, for which a patent, now expired, was obtained by Mr. Jewell, consists in conducting the sublimed vapours over an extensive surface of water contained in a covered cistern. The calomel thus obtained is a superior article, in an impalpable powder, propitious to its medical efficacy.
The presence of corrosive sublimate in calomel is easily detected by digesting alcohol upon it, and testing the decanted alcohol with a drop of caustic potash, when the characteristic brick-coloured precipitate will fall, if any of the poisonous salt be present. To detect subnitrate of mercury in calomel, digest dilute nitric acid on it, and test the acid with potash, when a precipitate will fall in case of that contamination. As it is a medicine so extensively administered to children at a very tender age, its purity ought to be scrupulously watched.
118 parts of calomel contain 100 of quicksilver.