=Chromic Acid=, now usually known as chromium-trioxide—CrO_{3}—is found in beautiful red needles, which are slightly deliquescent. According to the present accepted theory, water containing such crystals in solution contains the true chromic acid, H_{2}CrO_{4}, which cannot be produced as crystals. Both the crystals and a concentrated solution in water give up with facility a part of their oxygen, sometimes with explosions accompanied by the appearance of fire, and green chromium oxide, now called sesquioxide, Cr_{2}O_{3}, is formed. Bichromates of alkalies act in a similar manner, but in a lesser degree. If, for instance, a few drops of sulphuric acid be added to a solution of potassium bichromate, and then a little alcohol, or, instead of that, a piece of sugar be added and moderate heat applied, the solution will be violently agitated, its yellow-red colour will disappear, the bichromate be decomposed, and chromium oxide be formed, which, combining with the sulphuric acid, forms a solution of a violet green colour. If a sufficient quantity of sulphuric acid be present, the chromium sulphate and the potassium sulphate combine, forming chrome alum crystals, octahedron in form, similar to those of ordinary alum, but of a beautiful dark violet colour. The same are used in both the carbon and collotype processes. Bichromate of ammonium is easily decomposed by fire by heating it in a dry state. It will be understood that in consequence of the decomposition, or rather reduction, of chromic acid by organic matter, the latter is also decomposed and oxidised in a corresponding degree. The agent producing this effect may be either heat or light, or both together; but of special interest to the collotypist is the action of light upon different combinations of chromic acid, in the shape of alkali bichromates in conjunction with organic matters. =Glue or Gelatine=, in a solution containing about five per cent. of one of the bichromates spread upon glass plates, and exposed to light, will, where exposed, become dark brown, at the same time become insoluble in water, and also lose its power of absorbing the same. According to the researches of Dr. Eder, this brown substance consists of chromium chromate and unaltered gelatine. The oxidised portion of the latter remains soluble and may be entirely removed by treating with warm water. During the exposure to light formic acid is undoubtedly formed. Ammonium bichromate, with gelatine, is in the dry state much more sensitive than the potassium salt; it also decomposes more readily when heated in the gelatine solution, a matter which must not be lost sight of in the preparation of the collotype plate. =Isinglass=—a tolerably pure glutin. In conjunction with the bichromates it presents the same characteristics as gelatine.