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Gas Light

gelatine, water, pieces, process, bones and cold

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GAS LIGHT is used in photography, chiefly for exposing bromide prints in contact print ing. Its steadiness makes it preferable to ever-changing daylight. Gas light may also be used in the isochromatic process, especially for copying oil paintings, etc. It is but little used for portraiture, as the light obtained from a few burners is very non-actinic, and consequently the exposure would be very great. With a large number of burners, however, very good interiors may sometimes be made at night time.

GELATINE.—The word gelatine is derived from the Latin word gelatus, meaning frozen. It is so called because of the tendency the substance has of becoming solid. It is an animal gluten, obtained by boiling bones, hoofs, horns, and many other animal substances. Little is known of the chemical nature of gelatine, because it cannot be converted into vapor, and does not form well-defined compounds with other bodies. Its composition varies with the source from which it is obtained, but the following is about the average : Carbon 50'1 Hydrogen 6 6 Nitrogen 18.4 Oxygen 24'8 Sulphur. 0'1 The team gelatine, however, although usually applied to one variety of the substance, properly belongs to isinglass, glue, and other modifications of the same material. Vegetable jelly is also an analogous substance.

Gelatine is prepared for commercial purposes from a large variety of animal substances ; chiefly, however, from the softer parts of the hides of oxen and calves, and the skins of sheep and other animals. The parts used are usually the thin portions covering the belly, the ears, etc. The bones and other parts of animals are also used. The method usually adopted in making gelatine from skin parings or hide clippings is first to wash the pieces very carefully, after which they are cut into small strips and immersed in a weak solution of caustic soda for about seven or eight days. After this process of digestion is completed, the pieces of skin are transferred to revolving cylinders which are abundantly supplied with clean cold water. Here they are well washed and are then transferred, while still wet, to another chamber lined with wood. In this

they are bleached and purified by exposure to the fumes of burning sulphur. The next process is to give a final washing in cold water to remove the sulphurous acid. After this they are squeezed as dry as possible and removed to the gelatinizing vessels, which are large earthenware pots, enclosed in wooden cases, made perfectly steamtight. Water is also poured in with the pieces, and the temperature is kept very high by means of the steam passing through the cases sur rounding the pots. After a time the gelatine becomes completely dissolved out of the skin. It is then strained off, while hot and poured ont in thin layers. When set it is cut up into small oblong plates and laid on to netting, stretched horizontally, to dry. The cross markings observ able on these plates of gelatine are caused by the meshes of the nets. When string nets are em ployed, small pieces of the hempen fibre will often be found adhering to the gelatine. When dissolved it should be carefully filtered.

The process of obtaining gelatine from bones is carried out in this manner. Bones are first digested in cold dilute hydrochloric acid till the calcium phosphate and other salts are dis solved, leaving a residue of the same form, but of a soft, flexible character. This is termed ossein, and has the same composition as gelatine, into which it can be converted by prolonged boiling with water.

Another process consists in treating pieces of calf-skin by water alone, without the soda and sulphur processes. The pieces, after washing, are transferred at once to the steam-heated pots.

Gelatine contains about 15 to zo per cent. of water. It softens and swells in cold water, absorbing from five to ten times its own weight, but does not dissolve. Hot water dissolves it, and the solution gelatinizes on cooling, even if it contains but one per cent. It is also soluble in acetic, oxalic, hydrochloric, and sulphuric acids ; but is insoluble in alcohol, ether, and other organic liquids.

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