the Chromate of Iron, &c. fS 



we may avoid when the chromate of mercury is prepared 

 witli proper care. 



I{ is sufficient, in order to obtain the oxide of chrome' 

 very pure and of a very dne colour, to heat strongly in a 

 well luted earthen retoit the pure chromate of mercury, un- 

 til no more oxvgen is extricated, and to keep uj) the fire so 

 much (he longer in proportion to the depth of the shade 

 we wish to obtain : it seeins that there really exist two 

 kinds of oxide of chrome, for by heating it a very long 

 time the green is so weakened, that it passes to a dead- 

 leaf yellow. 



Combination of the Chromic Acid with Barytes. 



In-order to prepare the chromate of barytes, we em- 

 ploy with success chromate of potash well purified and 

 very neutral ; we mix with it nitrate of barvtes until no 

 more precipitate is produced : we must collect the latter, 

 decant the liquor, and wash several times, until it is en- 

 tirely freed from all extraneous saline j)articles. 



No harm is done by employing great quantities of watery 

 even warm, in order to wash the salt, for it is not very- 

 soluble. 



Analysis of the Chromate of Barytes, 



Five grammes of this salt, dissolved in the nitric acid and 

 precipitated by sulphuric acid, gave four grammes and four 

 tenths of sulphate of barytes. Admitting with M. Klaproth 

 68 parts of barytes in 100 parts of sulphate, we shall have, 

 in 100 parts of chromate of barytes, 62-2 of base, and 37*4 

 of acid. 



According to this account, the five grammes of chromate 

 of barvtes contain one gramme 87 centiemes of concrete 

 acid, and the latter convened to the state of green oxide by 

 a strong calcmation, is reduced to one gramme 56 cen- 

 tiemes, which gives a difference between the quantity of 

 oxvjren contained in 100 parts of oxide and 100 parts of 

 acid of 16-6, i. e. in order to convert 100 parts of oxide of 

 chrome into acid, we must combine with it 16*6 of oxygen. 



In order to effect the analysis of the chromate of barytes, 

 we must dissolve it in weak nitric acid, aided by a little 

 heat, and j)our into the solution sulphuric acid in excess; 

 we then wash the sulphate of barytes, and weigh it after 

 having dried and calcined it. 



With the same view, we may gently evaporate the !i- 

 quor to dryness, in order to drive off the nitric acid ; re- 

 ilis^lve the residue in water^ saturate it with ammonia,- 



and 



