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Again, should the impressioned plate be exposed too long over the vapor of bromine, the impression would be rendered wholly insensitive to the mercurial vapor. Hence this method is resorted to for restoring the sensibility of the plate when there is reason to believe that the impression would not be a desirable one; as, for example, if a likeness of a child be wanted, and it had moved before the plate had been sufficiently long exposed in the camera, the plate may be restored to its original sensitiveness by re-coating over bromine, as above, thus saving the time and labor of re-preparing the plate for the chemicals.
d. If by accident (we would not advise a trial to any extent of this), you should inhale a quantity of the vapor of bromine, immediately inhale the vapor of aqua ammonia, as this neutralizes the dangerous effect of the bromine vapor. every operator should be provided with a bottle of ammonia, as a little sprinkled about the chemical room soon disinfects it of all iodine or bromine vapor, and also tends to facilitate the operation in the camera.
History of Iodine.--This is one of the simple chemical bodies which was discovered in 1812 by M. Courtois, of Paris, a manufacturer of saltpetre, who found it in the mother-water of that salt. Its properties were first studied into by M. Gay Lussac. It partakes much of the nature of chlorine and bromine. Its affinity for other substances is so powerful as to prevent it from existing in an isolated state. It occurs combined with potassium and sodium in many mineral waters, such as the brine spring of Ashby-de-la-Zouche, and other strongly saline springs. This combination exists sparingly in sea-water, abundantly in many species of fucus or sea-weed, and in the kelp made from them. It is an ingredient in the Salt Licks, saline, and brine springs of this country, especially of those in the valley of the Mississippi. It is sparingly found in fresh-water plants, as well also in coal, and in combination with numerous other bodies.
Fermented liquors contain iodine; wine, cider, and perry are more iodureted than the average of fresh waters. Milk is richer in iodine than wine; independently of the soil, with which it varies, the proportion of iodine in milk is in the inverse ratio of the abundance of that secretion. Eggs (not the shell) contain much iodine. A fowl's egg weighing 50 gr. contains more iodine than a quart of cow's milk. Iodine exists in arable land. It is abundant in sulphur, iron, and manganese ores, and sulphuret of mercury: but rare in gypsum, chalk, calcareous and silicious earths. Any attempt to extract iodine economically should be made with the plants of the ferro-iodureted fresh waters. Most of the bodies regarded by the therapeutists as pectoral and anti-scrofulous are rich in iodine.
It is probably to the application of this body that we owe the discovery of the daguerreotype. There is no record of thep recise date when Daguerre commenced experimenting with iodine, but by the published correspondence between him and M. Neipce, his partner, it was previous to 1833. There is no doubt, however, that the first successful application was made in 1838, as the discovery was reported to the world early in January, 1839.
Preparation.--Iodine is mostly prepared from kelp, or the half vitrified ashes of seaweed, prepared by the inhabitants of the western islands, and the northern shores of Scotland and Ireland. It is treated with water, which washes out all the soluble salts, and the filtered solution is evaporated until nearly all the carbonate of soda and other saline matters have crystallized out. The remaining liquor, which contains the iodine, is mixed with successive portions of sulphuric acid in a leaden retort, and after standing some days to allow the sulphureted hydrogen, etc., to escape, peroxide of manganese is added, and the whole gently heated. Iodine distills over in a purple vapor, and is condensed in a receiver, or in a series of two-necked globes.
Properties.--Iodine is solid at the ordinary temperature, presenting the appearance of dark-grey or purple spangles, possessing a high degree of metallic lustre. It somewhat resembles plumbago, with which it is sometimes diluted, particularly when it is fine. Operators should endeavor to secure the larger crystals. It melts at 224.6 deg., forming a brown or nearly black liquid. It boils at about 356 deg., and emits a very deep violet colored vapor. It gives off a very appreciable vapor, sufficient for all purposes of forming the iodide of silver on the daguerreotype plate, at a temperature of 45 deg. or even lower. Iodine crystallizes readily. Every operator has found upon the side of the jar in his coating-box, perfectly regular crystals, deposited there by sublimation.
Water dissolves but a small proportion of iodine, requiring 7000 parts of water to dissolve one of iodine,
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