"It is well known that articles of silver plate become darkened by exposure to the fumes of sulphur, or to those of sulphureted hydrogen, of which minute traces are always present in the atmosphere. If the stopper of a bottle of sulphureted hydrogen water be removed, and a simply-fixed photographic positive suspended over it, the picture will lose its characteristic red tone, and become nearly black. The black color is even more intense than an experienced chemist would have anticipated, because analysis teaches us that the actual quantity of silver present in a photographic picture on paper is infinitesimally small; and it is well known that sulphide of silver, although of a deep brown color, approaching to black when in mass, exhibits a pale yellow tint in thin layers, so that a mere film of silver converted into sulphide possesses very little depth of color. To explain the difficulty it has been suggested that the toning action of sulphur on a red print is probably due to the production of a sub-sulphide possessing an intense colorific power, like the sub-oxide and sub-chloride of silver. When the toned picture is subjected to the further action of sulphur, is converted into the ordinary protosulphide of silver, and becomes yellow and faded." The toning baths following the sulphur method were principally mixtures of gold terchloride and hypo. This latter substance was found to be a solvent of certain silver compounds by the Rev. J. B. Reade, in 1839, Mr. Talbot having previously fixed his prints with common salt. Prints, too, were fixed first in some cases, and toned afterward, washing away the free silver being more or less practiced in the mixed hypo. and gold and the sulphur toning processes. When fixing was employed before toning, it was usual to soak washed prints in a twenty per cent. solution of hypo. for a period of ten minutes, or until the soluble silver salts were removed, the resulting color being a disagreeable yellowish-brown. To improve the result so obtained the prints were passed into a solution of-- Gold terchloride 10 grains. Water 20 ounces. When toning action quickly followed, the yellow color giving place to that of a dark sepia tint. From this stage to that of mixing these two substances together was only a natural sequence, and effected a diminution of gold to the extent of one-fourth, as will be seen by the following recognized formula: Hypo. 7 ounces. Water 20 " When dissolved, add-- Gold terchloride 5 grains. Dissolved in water 20 ounces. After mixing, a clear solution should result.