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water, cold, hot, grains, heat, plate, acid and solution

COFFEE PROCESS.—A number of modifications have been made of this process, the best being perhaps that of M. de Constant. Abney's description of this process is as follows: The collodion to be recommended for this process, according to M. de Constant, is ordinary collodion, with the addition of two grains of cadmium bromide to the ounce. If collodion be home-made the pyroxyline should be manufactured at a high temperature in the acids (see Pyroxyline), and may be known in commerce by its yellow appearance, and by being found to separate in short rather fibrous particles. The plate is given a substratum (see Collodion Process), it is then coated, and the film sensitized by placing in the silver bath for seven minutes in summer and ten in winter to convert the greater part of the bromide into the silver salt. It is then well washed.

The preservative is formed as follows: Solution No. i is allowed to cool in a well corked bottle. Both solutions should then be filtered and mixed.

Two applications of the preservative are necessary by immersing in a flat dish. The plate is then placed on end to dry upon blotting-paper to drain off the superfluous solution. It is then transferred to the drying box, and when dry the surface of the film should possess great brilliancy, and be free from fog or stains when viewed by transmitted light. If a cloudy aspect appears on parts of the film a heated flat-iron passed over it an inch or so from the surface will restore its brilliancy.

According to M. Constant the exposure required is about three or four times that necessary for wet plates.

In developing the plates should be placed in rain water for three or four minutes. They are then flooded over with Sat. sol. ammonium carbonate* 8 drops Water 4 drachms This quantity is sufficient for a whole plate. This is worked over the plate until the image begins to appear, and until there is no further action caused by it. The solution is then returned into the developing cup, in which has been previously dropped one or two drops of Pyrogallic acid 6o grains Alcohol 1 ounce The solution with this addition is again flooded over the plate as quickly as possible, its action being very rapid. The image now appears fully by reflected light, but hardly at all by transmitted light. When all detail in the shadow is brought out the image is intensified with Pyrogallic acid 2 grains Citric acid 2 to 4 grains Water i ounce But by this method the image will always appear transparent. To avoid this, M. de Con stant recommended the following before the final pyrogallic intensification: Ammonia sulphate of iron .45 grains

Copper sulphate 45 grains Citric acid .45 grains Water 3+ ounces Two or three drops of a zo-grain solution of silver nitrate can be added after the first application. On the second application the negative becomes of a color resembling that of a wet plate. The ordinary intensifier should be used after this. If the negative shows a tendency to become solarized, it should be fixed immediately, and the intensification done after fixing. These plates may also be developed with the ferrous oxalate or the alkaline developer. For fixing use hyposulphite of soda or a weak solution of potassium cyanide, with the addition of a few drops of acetic acid to prevent blistering.

COHESION.—The act of adhering or joining together. In chemistry, the force which unites two molecules of the same kind ; strong in solids, weak in liquids.

COLD.—It was formerly believed that cold was an entity, and that it could be reflected from polished surfaces like heat and light. This, however, is not the case. Cold is simply an absence of heat. It is essentially a relative term. Ice may be considered a hot substance when compared with frozen mercury, and a very hot substance when compared with solid carbonic acid. If we take three vessels, and pour hot water into the first, cold water into the second, and water of intermediate temperature into the third, and place one hand into the hot water and the other into the cold, we shall find on now placing both hands in the water of intermediate tem perature it will feel hot to the hand which has been in the cold water, and cold to the hand which was placed in the hot water; thus water at the same temperature may appear both hot and cold. Absolute cold would be the absolue zero of temperature, at which point matter would possess no heat at all. A substance is relatively cold when it possesses less of the motion called heat than the substance it is compared with. A hot substance—a red-hot suspended ball of metal, for in stance—gets colder and colder, because it radiates its heat into space; it loses molecular motion, and the more motion it loses the colder it is said to be. When it cools down to a temperature below that of our bodies, we call it cold to the touch, because it possesses less of the motion of heat than our nerves and abstracts heat from them, and this withdrawal of motion of the nerves produces the sensation cold. (Rodwell's Diet. Sci.)