A Manual Of Photography

98 HISTORY OF PHOTOGRAPHY. then carbonised in a closed box of cast iron, and, after cooling, passed betwixt two polished steel rollers, resembling a common copperplate printing press. The plate, after this process, is dipped into a strong solution of nitrate of silver, and instantly exposed to the action of the camera. The silver is, by the ac- tion of the rays of the sun, reduced into a perfectly metallic state, and the lights are expressed by the different density of the milk-white deadened silver ; the shadows by the black car- bonised plate. In a few seconds the picture is finished, and the plate is so sensitive, that the reduction of the silver begins even by the light of a candle. For fixing the image, nothing more is required than to dip the plate in alcohol mixed with a small quantity of the hyposulphite of soda, or of pure ammonia. These processes are given on the authority of the author ; but I have never been successful in producing a good result with either of them. The preparation of the plate requires the skill of an artist combined with the knowledge of a chemist; and even these are not always sufficient to ensure a perfect surface. The revival of the silver is not to be depended on : sometimes it does form a continuous sheet over the parts acted on by the light, but often it is only spangles ; and frequently a metallic arborescence will commence in the light parts, and run rapidly into the portions in shadow. The fact is, that light has the property of effecting the revival of the silver spread upon any carbonaceous body, but caloric having the same effect, and being indeed rather more active in the operation than light is, any slight increase of temperature produces a revival of the metal over the parts in shadow. Reference to the early volumes of Nicholson's Journal will afford ample evidence of these facts, which I have also recently proved. These volumes contain some papers by Count Bumford on the revival of gold and silver from their solutions, by heat and light, when spread upon charcoal or carbonaceous earth. This philosopher lias conclusively shown, that this revival is more dependent on the action of heat than light, which accounts, in some measure, for the apparent effect of candlelight. It is, however, possible that this process may, with some modifica- tions, become of importance. Section XII.—The Influence of Chlorine and Iodine in RENDERING SOME KINDS OF Wood SENSITIVE TO LiGHT. Having on many occasions subjected the simply nitrated pho- tographic paper to the influence of chlorine and iodine in close