CHROMIUM. 157 



a saturated solution of potassium dichromate, when chromium 

 trioxide separates in crystals : 



K 2 Cr 2 O 7 + H 2 SO 4 = K 2 S0 4 + H 2 O -f 2Ci-O 3 . 



Thus prepared, it forms crimson, needle-shaped crystals, 

 which are deliquescent, and very soluble in water ; it is power- 

 fully corrosive, and one of the strongest oxidizing agents ; the 

 solution in water has strong acid properties; it combines with 

 metallic oxides forming chromates and dichromates. 



Chromic oxide, Cr 2 3 (Sesquioxide of chromiuni), is obtained by 

 heating potassium dichromate with sulphur, when potassium 

 sulphate and chromic oxide are formed: 



K 2 O 2 0. -f S = K 2 SO 4 -f- O 2 O 3 . 



By washing the heated mass with water, the chromic oxide is 

 left as a green powder, which is insoluble in water and in acids; 

 it is a basic oxide combining with acids to form salts; it is used 

 as a green color, especially in the manufacture of painted glass 

 and porcelain. 



Chromic hydrate, Cr,6HO. A solution of potassium dichromate 

 may be deoxidized by the action of hydrosulphuric acid, sul- 

 phurous acid (or any other deoxidizing agent) in the presence 

 of sulphuric or hydrochloric acid: 



K 2 Cr 2 O 7 + 4H 2 SO 4 + 3H. 2 S = K 2 S0 4 + 7H 2 O -f 3S + Cr,3SO 4 . 



As shown by this formula, the sulphates of potassium and 

 chromium are formed and remain in solution, while sulphur is 

 precipitated, the hydrogen of the hydrosulphuric acid having 

 been oxidized and converted into water. 



By adding ammonium hydrate to the solution thus obtained, 

 chromic hydrate is precipitated as a bluish-green gelatinous 

 substance : 



Cr 2 3SO 4 -f 6XH 4 HO = 3[(XH 4 ) a S0 4 ] + Cr 2 6HO. 



By dissolving this hydrate in the different acids, the various 

 -salts, such as chloride, Cr 2 Cl 6 , sulphate, etc., are obtained. 

 Chromic sulphate, similar to aluminium sulphate, combines 

 with potassium or ammonium sulphate and water, forming 



