MANGANESE AND CHROMIUM 533 



if the gelatine mixture is made up with lampblack and, after the 

 coating has dried, is covered with a negative and exposed to light, 

 the parts which were protected from illumination may afterwards 

 be washed away, while the carbon print remains. The gelatine 

 layer can be transferred to wood or copper before washing. When 

 materials of different colors are substituted for the lampblack, 

 prints of any desired tint may be made by the same process. 



Sodium dichromate is used, instead of tan-bark, in tanning 

 kid and glove leathers. A reducing agent is employed to precipi- 

 tate chromic hydroxide Cr(OH) 3 in the leather. Its use dimin- 

 ishes the time required for the process from 8 or 10 months to a 

 few hours. The hide is a mixture of colloidal materials, and the 

 hydroxide is adsorbed. 



Chromic Salts: Chrome- Alum. The chromic salts are 

 made by reduction from chromates or dichromates. These latter 

 are derived from the anhydride CrO 3 , in which chromium is 

 sexivalent, while the chromic salts are obtained from the oxide 

 Cr 2 3 , in which the element is trivalent. 



When concentrated HC1 is heated with a dichromate, it is 

 oxidized to chlorine (compare p. 142) : 



K 2 Cr 2 7 + 14HC1 -> 2KC1 + 2CrCl 3 + 7H 2 + 3C1 2 . 



The most important chromic salt is chrome-alum, a double sul- 

 phate of potassium and chromium, which crystallizes from solu- 

 tion as beautiful purple octahedra K 2 SO4,Cr 2 (S04)3,24H 2 O (see 

 p. 470). 



Chromic Hydroxide Cr(OH) 3 and Oxide Cr 2 O 3 . When an 

 alkali is added to chrome-alum solution, chromic hydroxide 

 Cr(OH) 3 (green) is precipitated: 



Cr 2 (S0 4 ) 3 + 6NaOH -> 2Cr(OH) 3 j 



Chromic hydroxide is the substance precipitated in the leather in 

 chrome tanning. It is used also as a mordant in calico printing. 



