278 PRINCIPLES OF CHEMISTRY 



red heat for several hours, until the mass becomes yellow ; it then 

 contains normal calcium- chromate, CaCrO 4 , which is insoluble in 

 water in the presence of an excess of lime. 1 bis The resultant mass is 

 ground up, and treated with water and sulphuric acid. The excess of 

 lime forms gypsum, and the soluble calcium dichromate, CaCr 2 O 7 , 

 together with a certain amount of iron, pass into solution. The 

 solution is poured off, and chalk added to it ; this precipitates the 

 ferric oxide (the ferrous oxide is converted into ferric oxide in the 

 furnace) and forms a fresh quantity of gypsum, while the chromic acid 

 remains in solution that is, it does not form the sparingly-soluble 

 normal salt (1 part soluble in 240 parts of water). The solution then 

 contains a fairly pure calcium dichromate, which by double decom- 

 position gives other chromates ; for example, with a solution of potassium 

 sulphate it gives a precipitate of calcium sulphate and a solution of 

 potassium dichromate, which crystallises when evaporated. 2 



Potassium dichromate, K 2 Cr 2 7 , easily crystallises from acid solu- 

 tions in red, well- formed prismatic crystals, which fuse at a red heat 

 and evolve oxygen at a very high temperature, leaving chromic oxide 

 and the normal salt, which undergoes no further change : 2K 2 O 2 7 

 = 2K 2 Cr0 4 + Cr 2 O 3 + 3 . At the ordinary temperature 100 parts 

 of water dissolve 10 parts of this salt, and the solubility increases as 

 the temperature rises. It is most important to note that the 

 dichromate' does not contain water, it is K 2 CrO 4 + CrO 3 , the acid 

 salt corresponding to potassium acid sulphate, KHSO 4 , does not exist. 

 It does not even evolve heat when dissolving in water, but on the con* 

 trary produces cold, i.e. it does not form a very stable compound with 

 water. The solution and the salt itself are poisonous, and act as 

 powerful oxidising agents, which is the character of chromic acid in 

 general. When heated with sulphur or organic substances, with 

 sulphurous anhydride, hydrogen sulphide, &c., this salt is deoxidised, 

 yielding chromic compounds. 2 bis Potassium dichromate 3 is used in the 

 arts and in chemistry as a source for the preparation of all other 



1 bl " But the calcium chromate is soluble in water in the preseiice of an excess of 

 chromic acid, as may be seen from the fact that a solution of chromic acid dissolves 

 lime. 



2 There are many variations in the details of the manufacturing processes, and these 

 must be looked for in works on technical chemistry. But we may add that the chromate 

 may also be obtained by slightly roasting briquettes of a mixture of chrome iron and 

 lime, and then leaving the resultant mass to the action of moist air (oxygen is absorbed, 

 and the mass turns yellow). 



2 bis The oxidising action of potassium dichromate on organic substances at the 

 ordinary temperature is especially marked under the action of light. Thus it acts on 

 gelatin, as Poutven discovered ; this is applied to photography in the processes of photo- 



For Note 3 see p. 279. 



