The mixing, in equal or other proportions, of the potassium and ammonium salts has been recommended, but there is no real gain or sufficient reason for preferring any of these combinations to the ammonium salt alone. Besides the chromates of the alkalis, several other salts as well as acids are used in Collotype printing, although seldom, and then only in small quantities and for special purposes, such, for instance, as hardening the print layer, or for the purpose of facilitating the retention of moisture in the film. For the former purpose small quantities of chrome alum are sometimes added; while for the latter chloride of sodium or chloride of calcium are sometimes added to the chromated gelatine solution at the time of coating the plates, or are added as ingredients to the so-called etching or damping solution which is frequently applied to the plates during the printing. Glycerine answers for this purpose, and is to be preferred. It should be borne in mind that all additions to the film to facilitate its retention of moisture lessen its adhesiveness to its support. And with regard to the direct addition of the substances mentioned to the film during the preparation of the plates, it is not very apparent how such readily-soluble matters can remain in the film after the prolonged and careful washing the plates should be subjected to after exposure in the printing frame. On these grounds the addition of such substances to the damping solution is more rational and certainly more effective. =Alum= and =Chrome-Alum= causes the gelatine to set sooner without precipitation, and harden the layer considerably after drying, and although gelatine, as well as glue, always contains more or less chondrin, which is precipitated by alum, great care is necessary in the addition of the latter; and it is better that such addition should be made immediately before the hot solution be poured upon the plates, otherwise a precipitation will ensue, which, although very finely divided and at first scarcely visible, will become more apparent during the drying of the plates, and finally show in the picture in innumerable white spots. =Silicate of Soda= is almost indispensable as a substratum in the preparation of collotype plates, although many practical workers still adhere to the method originally proposed and practised by J. Albert, of Munich, who used as a substratum a chromated solution of gelatine and albumen, rendered either partially or wholly insoluble by exposure to light. The silicate is here considered as having about the consistency of syrup. If more fluid the adhesive power of the substratum will not realise expectations, and the plates will require very careful handling if even a few hundred impressions are required.