SULPHUR TONING. When an arp,entine photograph having the reddish tint produced by the combination of suboxide of silver with organic matter is placed in water containing a small quantity of sul phide of ammonium, the tint gradually changes from red to purple, and thence to green-yellow. This is called sulphur toning, the yellow substance being supposed to be sulphide of silver in an allo tropic state ; or it may possibly be a bisulphide of silver; or a double sulphide of silver and ainmonium. A similar result occurs when the print is placed in a bath of hyposulphite of soda containing unstable sulphur salts, or unstable compounds of sulphur and oxygen, or free sulphur in a nascent state or state of fine division, exhibiting a milky turbidity in the bath produced by the addition of an acid to it.
The chemistry of the sulphur toning of a red argentine photog,raph, althoug,h probably a very simple matter, has not yet been investigated by any chemist of eminence, and the subject cle,arly made out, although one of great importance, for thousand.s of pounds are annually wasted
by photographers in the production of perishable prints.
The ordinary fading of positives appears to be nothing more than the sulphur toning process carried to the yellow stage, in consequence of the presence of a destructive sulphur salt which cannot be removed from the paper. The combination of organic matter with the silver may have something to do with the result, and with the composition of the yellow substance. Photographs of a black tint, produced by development, and containing a much greater quantity of material than sun-prints, and that in a form much more nearly metallic, are found to be vastly more permanent then the latter.