The _sel d'or_ process followed, and was expected to give still better results. It was found, however, that the solutions would not keep; and as a considerable quantity of the gold salt was needed, it caused experimenters to search for a less expensive method. One decided point in its favor was the circumstance that prints suffered no loss of intensity during the operation, as they do in the case of all other toning methods. Briefly: the prints were well washed to extract free silver, and, after soaking five minutes in salt and water, they were passed into an alkaline solution composed of-- Liquid ammonia 60 minims. Water 20 ounces. Here they became very red. After washing in clean water the surface was flooded with a toning solution composed of Double hyposulphite of gold and sodium (_sel d'or_) ½ grain. Hypo 1 " Water 1 ounce. Upon the print assuming a purple-gray color it was withdrawn and fixed in a sixteen per cent. solution of hypo. to dissolve the unacted upon silver chloride. Gold, when in a fine state of subdivision, is of a rich purple color. The layer obtained by deposition upon a silver image is very finely divided; hence the color. The only object in continuing the toning action beyond the stage at which a good surface color has been reached is to obtain a deposit of sufficient density to completely neutralize the red color of the organic silver image beneath; therefore, it is preferable, in forming a judgment of toning action, to examine proofs by transmitted light rather than by reflected only. Before dealing with the various formulæ for alkaline toning I should like to step out of the golden track to say a few words on platinum tetracloride, PtCl_{4}.