A process which has been worked on a larger scale, is that of Dr. Heinzerling, introduced about 1878, with the usual promise of "complete revolution" in the leather trade; but which, in spite of the most determined and persevering efforts of the Eglinton Chemical Company, who own the English patent, has failed to take any very prominent position in commerce. The tanning materials employed are alum and salt, with a varying proportion of potash, soda, or magnesia bichromate. These salts have a very marked hardening effect on animal tissues, and, when mixed with gelatin and exposed to light or acted on by acids, become reduced, and at the same time render the gelatin insoluble even in hot water, a property which is made useful in many photographic processes. This is probably due to the formation of salts of chromium, which, as has been stated (p. 219) have a similar tanning effect, but perhaps more powerful, than those of alumina. However this may be, the effect of potash bichromate when exposed to light with gelatin, differs from that of the addition of chrome salts ready formed, the gelatin in the first case becoming incapable of even swelling in hot water, while in the second, though rendered insoluble, it becomes soft and swollen. The use of potash bichromate in tanning had been previously patented by Cavalin, who used it in conjunction with alum and salt, and with the addition of a portion of green vitriol, to give the leather a colour more similar to that of bark-tanned. Dr. Heinzerling uses metallic zinc in the salt and alum solution to assist in the precipitation of amorphous alumina on the hide-fibres. The same material was used in a similar way by Jennings (No. 2295, 1861), but with the object of whitening the goods. Yellow or red prussiates of potash (potassic ferrocyanide or ferricyanide), are also sometimes mixed with the solution, in order to enable the leather to be blacked with iron-liquor, with which they produce prussian blue. To fix the tannage on the fibre, and prevent its washing out, the use of barium chloride, lead acetate, and of soap solution is claimed; the latter having been also patented for similar purposes by Knapp, and subsequently by Jennings and others. In order to render the leather waterproof, it is finally saturated with solutions of paraffin, stearin, and other fats and hydrocarbons (resin is employed, though not named in the patent), in petroleum spirit and similar solvents.