JI!|

176

depend upon obtaining the same results with the same materials,owing principally to the difficulty of preserving the solution atauniform strength. Liquid chlorine is necessary—the applicationof dry chlorine will not produce the same results, and the volatilechlorine is continually escaping from the water.

Niepee de St.-Victor has made many experiments to producethe colours upon salts of silver and copper spread on paper, butwithout success; the metallic plate appears absolutely necessary,and the purer the silver the more perfect and intense is theimpression. The following is recommended as the most effectualmode of manipulating. The silver plate is highly polished withthe best tripoli powder and ammonia; being perfectly cleaned itis connected with the battery, and then plunged into the bathprepared in any of the ways stated. It is allowed to remain inthe bath for some minutes, taken from it, washed in a largequantity of water, and dried over a spirit-lamp. The surfacethus produced is of a dull neutral tint, often almost black; thesensibility of the plate appears to be increased by the action ofheat, and when brought by the spirit-lamp to the cerise redcolour it is in its most sensitive state. The sensibility, how-ever, of the plates is low, two or three hours being requred toproduce a decided effect in the camera obscura. The name ofHeriocEroMES has been given to these naturally colouredphotographs, some of which, the personal gift of the inventor toMyr. Malone, I have inspected. These, when I first saw them,were perfectly coloured in correspondence with the drawings ofwhich they were copies; but the colours soon faded, and it doesnot appear as yet that any successful mode of fixing the colourshas been discovered.

SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY.