the Chromate of lion, ^c. " 23 



. I have attentively examined this precipitate, in order to 

 become better acquainted with its nature and properties. 



When treated cold with caustic alkali it dissolves, and 

 communicates a fine green colour to the liquor : a red pow- 

 der remained which presented all the characters of oxide of 

 mercury at the maximum. The alkaline solution, being fil- 

 tered and subjected to ebullition, deposited a great quantity 

 of green flakes of oxide of chrome, and preserved a line 

 golden yellow colour: this was chromate of potash. 



Having d'iluted in water a portion of the precipitate ob- 

 tained by the alkali of the above mother waters, I remarked 

 that it was composed of two different substances ^ the first 

 was flaky and light; the second, which always occupied the 

 bottom of the mixture, was formed .of small crystals of a 

 violet brown. These last presented the following proper- 

 ties: 1st, When thrown on burning charcoal, they are 

 entirely volatilized and condensed in small purple needles 

 on such cold bodies as are exposed to their fumes. 2d, 

 When heated more slowly in a retort, they furnish mer- 

 cury, and leave green oxide of chrome as a residue : they 

 are dissolved in weak nitric acid, communicating to it a 

 fine yellow colour: if we pour into this solution nitrate of 

 mercury at the minimum, common chromate of mercury is 

 precipitated. 3d, When we treat this substance by^a caustic 

 alkali, the latter acquires a yellow colour, and a red powder 

 remains which is oxide of mercury at the 7naximum^ while 

 the common chromate of mercury givt;s black oxide by the 

 same process. 



The greenish precipitate obtained by the saturation of th^ 

 mother waters, by means of an alkali, contains therefore 

 green oxide and chromate of mercury at {hit maximiim. 



From what is above stated, we- may easily explain what 

 takes place when the mother waters, although containir>g 

 mercury in excess, nevertheless give a precipitate of chro- 

 mate of mercury by the addition of fresh nitrate. The reason 

 of this is, that although the nitrate of mercury at the maxi^ 

 mum precipitates the chromate of potash, it requires but a 

 very small quantity of acid in order to be dissolved : this in 

 cjined me to think formerly that this' precipitation had not 

 taken place, because t he mercurial solution, containing always 

 an excess of acid, and being rarely at the perfect minimum^ 

 the chromate at the maximum, on account of an excess of 

 acid, is kept in solution ; but when we add a fresh quantity 

 of nitrate of mercury, the portion at the minimum takes off 

 the chromic acid from the red oxide which is deposited : if 



B 4 on 



