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these card-boards furnishes, by radiation, to the metal the vapour ofiodine, while the other returns to the cotton that which it had lost. Itsuffices to turn the plate from time to time, in order that the operationmay go on with equal rapidity. A plate of glass is placed upon theupper card-board, where it is not operated on. The plate is sustaineda little above tbe charged cotton, by frames of hard wood, varnishedwith gum lac. By increasing the distance between the cotton and theplate, or the contrary, we are enabled to suit the arrangement to thetemperature of the season, and thus always operate with facility andpromptitude. M. Seguier also states that a single scouring with tripoli,moistened with acidulated water, is sufficient to cleanse the platesthoroughly, and does away with the tedious process of scouring withoil, and afterwards the operation of heating the tablet over a spiritlamp. M. Soliel has proposed the use of the chloride of silver to de-termine the time required to produce a good impression on the iodatedplate in the camera. Ilis method is to fix at the bottom of a tube,blackened within, a piece of card on which chloride of silver mixedwith gum or dextrine* is spread. The tube thus disposed is turnedfrom the side of the object of which we wish to take the imago, andthe time that the chloride of silver takes to become of a greyish slatecolour, will be the time required for the light of the camera to producea good effect on the iodated silver.
b. Methods of fixing the Daguerreotype Pictures.
Various methods have been tried for more perfectly fixing the Da-guerreotype pictures, but most of the proposed plans impair the beauti-ful delicacy of the design. The object desired, is to occasion a moreperfect adhesion of the mercury and silver, and this is said to beeffectually done by M. II. Fizeau , whose method is as follows:—Aboutfifteen grains of chloride of gold is dissolved in a pint of puro water;thrice that quantity of the hyposulphite of soda is dissolved in a likequantity of water: the former solution is then poured into tho latter,stirring all the while. Tho mixed liquor, at first slightly yellow, soonbecomes perfectly limpid. It appears to bo a double hyposulphite ofsoda and gold rather than a chloride of sodium, which appears to actno part in the operation.
After the plate has undergone the usual photographic processes, it iswashed in alcohol and water. It is then placed upon an iron frame,and covered with some of the solution of the salt of gold and soda.Heat is applied by means of a powerful lamp, and the impression be-comes clear, and acquires in a few minutes great force. When this isproduced, the liquid must be poured off, and the plate washed anddried. It appears that in this operation silver is dissolved, and gold isprecipitated on the silver and on the mercury. The silver is turnedslightly brown by the thin layer of gold which covers it, by which the
A kind of gum procured from starch and other similar substances.