Black Oxide of Mercury . 227 



From the clear fluid a black precipitate is gradually 

 throttn down by means of ammonia, and more ammonia 

 must be dropped into it till no more precipitate is formed. 

 By these means the precipitate from the becrinnincr to the 

 end of the operation will be uniformly black, and more 

 black oxide will thus be obtained than by the methods hi- 

 therto employed. Should the precipitate be of a brighter 

 colour towards the end of the operation, which, however, 

 will not be the case when the above prescription is closely 

 followed, the fluid must be separated from the precipitate 

 before the precipitation is completed, and the remaining 

 fluid must then be precipitated by itself. The last precipi- 

 tate is to be kept for a future preparation, or for any other 

 use. 



Wash the black precipitate which has been obtained, in 

 a glass vessel with cold distilled water, and place it on a 

 paper filter, where in a few hours it will be freed from the 

 greater part of its moisture. Then twist together the ex- 

 tremities of the filter, so that none of the precipitate can 

 escape from it ; and having wrapped it up in a few folds of 

 blotting-paper, subject it to a press, or place weights over 

 it : but this must be done with great care, lest the paper 

 should burst : it must, however, be well pressed at last, in 

 order to free it as much as possible from water. 



When this is done, take the filter from the press ; and 

 having freed it as speedily as possible from the wrappino- 

 paper, spread it out carefully on some solid supporter, ex- 

 tending the dry precipitate in a thin stratum. Suffer it to 

 dry completely in a temperature which must not exceed 70"* 

 of Fahrenheit. By these means you will obtain a prepara- 

 tion which will stand every proof in regard to its purity. 



I shall conclude this account with a few observations on 

 the process in general, and on some individual parts of it. 



1st. The principle on which this method is founded is 

 experience, which shows that acids in general are capable 

 ot taking up any metallic oxide, according as it is more or 

 less oxidated. 



2d. When mercury and nitrous acid exposed to heat 

 are sufl'ered to exercise an action on each other till the dis- 

 engagement of air-bubblts ceases, a combination will be 

 obtained in which the oxide of mercury has experienced 

 a stronger degree of oxidation than is necessary for black 

 oxide of mercury, as this solution is precipitated white 

 by aunnonia. But if mercury not oxidated remains in 

 contact with the solution during the continuance of the ex- 

 posure to heat, a part of the oxygen passes from the dis- 

 1' tj solved 



