A Manual Of Photography

176 SCIENTIFIC INVESTIGATIONS ON PHOTOGBAPHY. depend upon obtaining the same results with the same materials, owing principally to the difficulty of preserving the solution at a uniform strength. Liquid chlorine is necessary the application of dry chlorine will not produce the same results, and the volatile chlorine is continually escaping from the water. Niepce de St.-Victor has made many experiments to produce the colours upon salts of silver and copper spread on paper, hut without success ; the metallic plate appears absolutely necessary, and the purer the silver the more perfect and intense is the impression. The following is recommended as the most effectual mode of manipulating. The silver plate is highly polished with the best tripoli powder and ammonia ; being perfectly cleaned it is connected with the battery, and then plunged into the bath prepared in any of the ways stated. It is allowed to remain in the bath for some minutes, taken from it, washed in a large quantity of water, and dried over a spirit-lamp. The surface thus produced is of a dull neutral tint, often almost black ; the sensibility of the plate appears to be increased by the action of heat, and when brought by the spirit-lamp to the cerise red colour it is in its most sensitive state. The sensibility, how- ever, of the plates is low, two or three hours being requred to produce a decided effect in the camera obscura. The name of Heliociibomes has been given to these naturally coloured photographs, some of which, the personal gift of the inventor to Mr. Malone, I have inspected. These, when 1 first saw them, were perfectly coloured in correspondence with the drawings of which they were copies; but the colours soon faded, and it does not appear as yet that any successful mode of fixing the colours has been discovered.