$84 PRINCIPLES OF CHEMISTRY 



Chromic oxide, Cr 2 3 , rarely found, and iri small quantities, in chrome' 

 ochre, is formed by the oxidation of chromium and its lower oxides, by*' 



does not exceed 85, a violet solution of chrome alum is obtained, but if the tempera- 

 ture be higher, a solution of the same alum is obtained of a green colour. As chrome 

 alum requires for solution 7 parts of water at the ordinary temperature, it follows that il 

 a somewhat strong solution of potassium dichromate be taken (4 parts of water and 1J 

 of sulphuric acid to 1 part of dichromate), it will give so concentrated a solution 

 of chrome alum that on cooling, the salt will separate without further evaporation. If 

 the liquid, prepared as above or in any instance of the deoxidation of chromic acid, 

 be heated (the oxidation naturally proceeds- more rapidly) somewhat strongly, for in- 

 stance, to the boiling-point of water, or if the violet solution already formed be raised to 

 the same temperature, it acquires a bright green colour, and on evaporation the 

 same mixture, which at lower temperatures so easily gives cubical crystals of chrome 

 alum, does not give any crystals whatever If the green solution be kept, however, for 

 several weeks at the ordinary temperature, it deposits violet crystals of chrome alum. 

 The green solution, when evaporated, gives a non-crystalline mass, and the violet 

 crystals lose water at 100 and turn green. It must be remarked that the transition of 

 the green modification into the violet is accompanied by a decrease in volume (Lecoq de 

 Boisbaudran, Favre). If the green mass formed at the higher temperature be evaporated 

 to dryness and heated at 30 in a current of air, it does not retain more then 6 equi- 

 valents of water. Hence Lbwel, and also Schrbtter, concluded that the green and violet 

 modifications of the alum depend on different degrees of combination with water, which 

 may be likened to the different compounds of sodium sulphate with water and to the 

 different hydrates of ferric oxide. 



However, the question in this case Is not so simple, as we shall afterwards see. 

 Jtfot chrome alum alone, but all the chromic salts, give two, if not three, varieties. At 

 least, there is no doubt about the existence of two a green and a violet modification. 

 The green chromic salts -are obtained by heating solutions of the violet salts, the violet 

 solutions are produced on keeping solutions of the green salts for a long time. The con- 

 version of the violet salts into green by the action of heat is itself an indication of the 

 possibility of explaining the different modifications by their containing different propor- 

 tions (or states) of water, and, moreover, by the green salts having a less amount of 

 water than the violet. However, -there are other explanations. Chromic oxide is a base 

 like alumina, and is therefore able to give both acid and basic salts. It is supposed that 

 the difference between the green and violet salts is due to, this fact. This opinion of 

 Kriiger is based on the fact that alcohol separates out a salt from the green solution 

 which contains less sulphuric acid than the normal violet salt. On the other hand, 

 Lbwel showed that all the acid cannot be separated from the green chromic salts by 

 suitable reagents, as easily as it can be from the same solution of the violet salts ; thus 

 barium salts do not precipitate all the sulphuric acid from solutions of the green salts. 

 According to other researches the cause of the varieties of the chromic salts lies in a 

 difference in the bases they contain that is, it is connected with a modification of the 

 properties of the oxide of chromium itself. This only refers to the hydroxides, but as 

 hydroxides themselves are only special forms of salts, the differences observed as yet 

 in this direction between the hydroxides only confirm the generality of the difference 

 observed in the chromic compounds (see Note 7 bis). 



The salts of chromic oxide, like those of alumina, are. easily decomposed, give basic 

 and double salts, and have an acid reaction, as chromic oxide is a feeble base. Potas- 

 sium and sodium hydroxides give a precipitate of the hydroxide with chromic salts, 

 CrX 3 . The violet and green salts give a hydroxide soluble in an excess of the 

 reagent ; but the hydroxide is held in solution by very feeble affinities, so that it is 

 partially separated by heat and dilution with water, and completely so on boiling. 

 In an alkaline solution, chromic hydroxide is easily converted into chromic acid 

 by the -action of lead dioxide, chlorine, and other oxidising agents. If the chromic 

 oxide occurs together with such oxides as magnesia, or zinc oxide, then on precipitation 



