=Bichromate of Potassium= is found in the well-known rich orange-red crystals. It is readily soluble in cold water (1.10). It may be easily recrystallised and purified readily out of either a hot, warm, or cold solution, and forms in thin layers of fine crystals. Therefore, in using it for collotype, care is needed not to exceed the strictly necessary proportion, as the plate may be hopelessly spoiled by the partial crystallisation of the salt, which will probably show during the process of drying. The maximum addition of bichromate of potassium in proportion to the weight of the gelatine may differ according to the peculiarities of the latter. Dr. Eder considers about 3/10 to 4/10 by weight of this salt in a dry state may be added to a gelatine, which will absorb from six to ten times its weight of water, without fear of crystallisation. There are, however, many gelatines that are incapable of working with more than 2/10. Rapid drying of the plate in a current of air, and at a temperature of 40° C., will, to some extent, obviate this objectionable feature. By reason of its comparative insensitiveness to light and this facility of crystallisation, and notwithstanding its comparative low price, it is now far less used in collotype than =Bichromate of ammonium=, the crystals of which somewhat resemble those of the potassium salt. In colour they are of a browner hue, and readily distinguishable, since when heated on platinum deflagrates, emitting sparks and leaving a residuum of very finely-divided oxide of chromium. The ammonium salt does not, however, so readily crystallise on account of its considerably greater solubility in water compared with bichromate of potassium. By comparison, 12 parts of the ammonium salt are equal to 15 parts of that of potassium. At least 1/3 to 1/4 more of the former may be used without disadvantage or fear of it crystallising out; it will, on the contrary, improve the quality of the film or layer. The ammonium salt will also be found comparatively more soluble in hot water, but care must be exercised that the solution never reaches the boiling point, neither should the heat of the drying box ever exceed 50° to 60° C., or the films will present a pale brown appearance, and will prove alike insensitive and partially insoluble. The single and tri-salts, offering no advantages over those already described, may be dismissed without further consideration. It may be remarked for non-chemists that the addition of any acid to the chromated gelatine solutions inclines to the formation of trichromates of the alkalis; and, on the contrary, the addition of free ammonia, or any other free alkali, produces simple chromates. The addition of ammonia to the chromated solution has been strongly advocated in many quarters, but as the result of numerous experiments, it may be definitely stated that the supposed advantages are more than counter-balanced by the numerous disadvantages arising from such addition.