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Gratime

silver, green, chloride, weight and albuminate

GRATIME is the standard unit of French measures of weight, and is the weight of a cubic centimetre of distilled water at 0 deg. centigrade (32 deg. Fahrenheit). It equals 15.42348 grains. The English equivalents for the other French weights corresponding to the number of grammes they contain is easily found thus :— In abbreviating the word gramme Gm. is now generally adopted to distinguish it from gr. or grain.

Gramme quantity of an elementary substance, such that the number of grammes weight is the same as the atomic number of the element— e.g., 4o grammes of calcium (Ca = 40).

Gramme number of grammes weight of a substance, elementary or com pound, equal numerically to the quantity of that substance, which is chemically equivalent to unit weight of oxygen.

Gramme quantity of a substance, either elementary or compound, such that the number of grammes and the molecule weight are numerically the same.

GRANULATION.—The process of becoming granular or formed into grains. Zinc and tin may be granulated by pouring the melted metal into cold water.

The term is also applied in photo-mechanical printing to the processes of breaking up the image into a granular state so that the half-tones of the picture may be represented although only one colored ink is used.

GRAPE SUGAR.—See Sextrose.

GRAPHITE.—An iron-black mineral commonly known as blacklead, and largely used in the manufacture of pencils. Ground up with diluted negative varnish it gives a dull black coating. Carbon in another form, i.e., lampblack is largely used as a color in carbon printing, and Woodburytype, but usually with some other tint to make the color more agreeable. (See Lampblack.) GRAPHOSCOPE.—An optical apparatus for magnifying photographs or other pictures.

GREEN COPPERAS.—See Ferrous Sulphate.

GREEN F00.—(See Fog.) GREEN GLASS.—Glass colored green has been discovered to be of great service in print ing upon chloride of silver paper, such as albumenized paper, etc. It is found that by laying a piece of green glass over the printing-frame, and allowing the light to shine through it before reaching the negative, a far more vigorous print is obtained if a weak negative be employed. The reason of this is that, when the paper is floated on the nitrate bath, two compounds are formed, albuminate and chloride of silver. The silver chloride is insensitive to green light, but the silver albuminate is sensitive. Now the more albuminate of silver contained in the sensitive film, the greater will be the contrast when printing with a weak negative, and if we rob the light from the silver chloride, and allow it to act on albuminate of silver, we obtain the same result. Green glass has this effect by absorbing those rays which have an actinic effect upon the chloride of silver, and allowing those to pass through which will affect the silver albuminate, and by this means much more rapid gradation is obtained, although, of course, the exposure is considerably increased. It also exercises an effect upon the subsequent toning operation. (See Toning.) With a gelatino-citro- chloride paper the same effect, but in a greater degree, is obtained, as the citrate of silver is more sensitive than the chloride. With Aristo papers vigorous prints may be obtained from weak negatives, and from the very weakest by employing green-colored glass.