282 Notices of the Labours of Continental Chemists. 



precipitated, which is insoluble in boiling or cold water, hydro- 

 chloric, sulphuric and nitric acids ; it is not altered by am- 

 monia, but it is decomposed by boiling with caustic potassa ; 



formula, Cr S 3 , K S. It is easily decomposed by heat. It is 

 evident therefore that the chrome alum can exist in three dif- 

 ferent isomeric modifications. When chrome alum is heated at 

 200° C. as long as water is driven off, a " difficultly soluble" 

 green modification is formed ; it contains 2 atoms of water ; it 

 is insoluble in cold water, and also in sulphuric and hydro- 

 chloric acids, but it is dissolved by continued boiling with 

 water ; decomposition is also effected by boiling ammonia. 

 When heated to 300°-400° it passes into the " insoluble" 

 modification, its dark green colour changes to light green, and 

 it has lost all its water. This anhydrous salt differs from that 

 obtained with sulphuric acid, inasmuch as by long boiling with 

 water, sulphate of potassa is dissolved and insoluble sulphate 

 of chromium remains behind. 



In vol. xliii. of the same Journal, p. 513, Schrotter has de- 

 scribed some sulphates of chromium. Cr S 2 is obtained by 

 adding as much hydrated oxide of chromium to sulphuric 

 acid as it can take up when kept boiling for a long time ; it is 

 not crystallizable, and forms a green mass on evaporation ; on 

 the addition of water a light green powder separates, which is 



Cr 3 S 2 + 1 2 aq. If a solution of the first salt be heated with 

 excess of sulphuric acid the green colour disappears and a 

 peach red precipitate is formed, which is not soluble in water, 

 and is not decomposed by acids or ammonia, but easily by 

 caustic potassa or soda. A solution of this salt may be 

 obtained by dissolving eight parts of oxide in nine parts of 

 English sulphuric acid; alcohol does not precipitate the 

 fresh solution ; if it be allowed to stand several weeks it 

 forms a greenish blue crystalline mass, which dissolved in water 

 forms a dark blue (by transmitted light ruby red) solution. 

 Out of this alcohol precipitates a light violet-coloured crystal- 

 line salt, Cr S 3 + 15 aq, which is easily soluble in water, be- 

 comes green when heated to 100°, and loses 10 atoms of water. 

 For preparing the chrome alum, Schrotter proposes to pass 

 sulphurous acid into a solution of one atom of bichromate of 

 potassa and one atom of sulphuric acid, as long as it is ab- 

 sorbed, the mixture being kept cool. He has also prepared 

 the ammonia and soda chrome alum ; they both contain 24 

 atoms of water. [The ammonia compound was prepared by 

 Mr. Warington several years ago {vide Turner's Chemistry).; 



