the Chromate of Iron, ^c, 27 



after w?ilch we must boil in order to drive off ibe super- 

 abundant sulphuretted hydrosien, filter in order to separate 

 the sulphur, pour into the filtered liquor some drops of 

 caustic potash, and thus obtain the green oxide of chrome ; 

 filter again, wash caretuliy, dry and calcine carefully. Oil 

 4ddingto the quantity of oxide obtaine/J the l6*6 centiemes 

 of its weight, we have the quantity of chromic acid which 

 any given salt comained. We may easily see that in this 

 operaiion the sulphuretted hydrogen makes the chromic 

 acid pass to the state of green oxide, which the excess of 

 acid takes up again in proportion as it is precipitated : if 

 some portions of it remain, we succeed in separating it from 

 the sulphur by ebullition, which serves at the same time to 

 drive t»fi' the sulphuretted hydrogen. The potash after- 

 wards added only serves to decompose the salt of chrome- 

 ■\yhich is formed. 



Action of Sulphurous Acid on Chromic Add, 



IF into chromic acid we pour a great quantity of sulphu- 

 rous acid, its ycllowish-red colour passes to a dirty brown; 

 and if" at this period we pour caustic alkali into the liquor, 

 we obtain a reddisfi-brown precipitate, which is dissolved 

 in the acids. 



A greater quantity of sulphurous acid immediately changes 

 the red colour of the chromic acid to a pale green. Thus 

 it should seem that two kinds of oxide of chrome exist, 

 but wUich scarcely differ in the quantity of oxygen*. 



Action of the Acids on Oxide of Chrome. 



In general the oxide of Qhrome obtained by the calcina- 

 tion of the chromate of mercury is attacked by the acids 

 with very great difficulty : we at length succeed, however, 

 in dissolving it; but in order to form combinations we 

 made use of the oxide obtained from the decomposition of 

 the chromate of potash by means of the hydro-suiphuret, 

 according to the process above described. We have seen 

 that this oxide, when recently precipitated, is dissolved 

 with the greatest facility even in trie weakest acids. 



The s«jiphate of chrome presented nothing reuiarkable; 

 we merely observed that it is easily decomposed by heat ; 

 for, when slightly calcined, it is no longer redissolved in 

 watt.-. 



• We obtain an oxide similar to that which is produced by sulphurous 

 acid in the former case, by passing oxygeaizcd muriatic acid into an acid 

 sblutiun of green oxide of chrome, and into v.Jiich we gradual v pour a 

 little pt»ta;sh. ...'.' 



- The 



