ALICALINE SOLUTION Sodium carbonate (crystals) 125 grams Water 500 c. c.
For a developer, 30 c.c. of pyro solution and 30 c.c. of alkaline solution are put into 185 to 305 c. c. of water. It was found about this time to obtain the best results on plates having had instantaneous exposures, that potassium carbon ate as an alkali superseded soda, and this is largely used at the present time, in combination with soda, particularly for the development of shortly-timed plates. In 1889 and since then the new coal tar developing agents were introduced under the name of eikonogen, metol, glycin, ortol, etc. They largely take the place of pyro galhc acid. The fixing agent for dissolving out the creamy unacted-upon film after develop ment is hyposulphite of soda.
Printing methods in photography have been as varied and their improvements as great as in the case of the negative. At first prints were made on plain silver chloride paper, and when the ammonia-nitrate was substituted for the plain nitrate, some were made that are not ex celled by anything at the present day. The desire for detail, however, brought into use albummized paper, which not only came into universal use, but held its sway until com paratively recent times. About a decade ago it was displaced by paper coated with a chloride emulsion, a highly glossy family, generally known as the uaristo.° This still continues in use and is largely used in prints intended for process reproduction, on account of the clear rendering of fine details. For truly pictorial work methods which give or plain paper prints are preferred. The principal or the most generally employed, are the °carbon° and the °platinum"' methods, both introduced in the '60s' though both lay dormant for many years.
The carbon, probably the best of all printing methods, although platinum is a close second, is more of a mechanical than a chemical proc ess. It uses a paper coated with bichromated gelatine, colored with finely divided carbon or other pigment. This is exposed under a nega tive, and wherever light has reached the atissue,') as it is called, and just in proportion to the quantity or intensity of that light, the gelatine becomes insoluble. Immersed in warm
water, the soluble parts of the tissue (those protected by the opaque or semi-opaque parts of the negative, and consequently the lights of the picture) soften and are washed away. Who first proposed carbon is uncertain. Fargier, in France, was early in the field, but to Swan of England is due the credit of first making it a practical process; although Blair of Perth, Scotland, was the first to recognize the crucial part of it — the necessity for exposing through the back of the tissue, or in other words, veloping from the side opposite to that which was exposed. Platinum was introduced by Willis of England in 1874, and is based on the fact that a platinum salt is reduced to its metal in the presence of potassium oxalate and a ferrous salt of iron. At first the paper was coated with the ferric salt of iron and exposed to light under the negative, the light changing the ferric to the ferrous salt. Development was effected by a hot solution, a mixture of the oxalate and the platinum salts; but more recently the platinum has -been mixed with the iron, and development carried on in a cold solution of potassium oxalate.
During recent years two modifications of the carbon printing process have come into pretty general use, especially among picto rialists ; and "ozotype.2 They are simpler than the original method, do not reverse the image, need no transfer and are supposed to give greater control.
In the gum-bichromate process, paper is evenly coated with a suitable mixture of gum arabic, coloring matter and potassium bichro mate, and dried. It is printed under a negative in the ordinary way and developed by floating on water of suitable temperature, assisted by brush action, letting the water fall in streams and sometimes mixed with sawdust to assist the removal of color from the lights.