248 Mr. Donovan on the [Oct. 



should be made in the cold ; and secondly, that there should be 

 some mercury present which the acid could not dissolve. But 

 it will be found that neither of these circumstances is necessary. 

 If heat be used, there should not, however, be a great excess of 

 concentrated acid. 



24. When the crystals of nitrate are purified by a second 

 crystallization, and are decomposed by salt, potash will not 

 precipitate a particle of red oxide, or any other oxide, from the 

 residual liquor ; and when lime-water is added to a solution of 

 this nitrate, there is no deposition of a yellow oxide. The yellow 

 precipitate obtained by the writer in liees's Cyclopaedia, above 

 referred to, was not an oxide, but a subnitrate mixed with sub- 

 oxynitrate, as will be explained hereafter. 



25. To ascertain the influence of heat, in determining the 

 oxidation of mercury by nitric acid, I dissolved b'O gr. of the 

 metal in two measured drachms of nitric acid (T275) by heat. 

 The solution was mixed with an excess of solution of muriate of 

 soda. The calomel, when dried, weighed 14 gr. The filtered, 

 liquor was precipitated by an excess of pure potash ; the red 

 oxide, when dry, weighed 50 gr. The same quantities were 

 treated in the same manner, but that the solution was effected 

 in the cold. The calomel weighed 35 gr. the red oxide 28 gr. 

 In both cases there was the loss of a minute quantity of mercury. 



26. Muriate of soda mixed with solution of nitrate, prepared 

 in the cold, occasions an effervescence, owing to the escape of 

 nitrous gas, which the solution held dissolved ; but if the solu- 

 tion be previously heated, there is no effervescence. 



27. The crystals of nitrate of mercury have been stated by 

 some writers as deliquescent ; but in this case there is always a 

 large portion of oxynitrate present. The nitrate is a permanent 

 salt, requiring a tolerably large quantity of water for solution. 

 These crystals have been also supposed to dissolve in water 

 without decomposition ; but this is not the case, unless the water 

 be acidulated with nitric acid. 



28. It has been stated by chemists, that when nitrate of 

 mercury is exposed to air, the salt becomes yellow, and the metal 

 passes to a higher degree of oxidation. I find that this change 

 of colour does not depend on the absorption of oxygen, but on 

 the dissipation of the acid, a subnitrate being the result; and 

 this never happens but when the crystals have been formed in a 

 solution that has but a small excess of acid. The oxide con- 

 tained in this yellow salt is the black. 



29. When nitrate of mercury is mixed with water, a white 

 precipitate appears, which has been considered subnitrate of 

 mercury, and to a certain extent is so. When washed with 

 large quantities of cold water, it becomes yellow ; and if w itli 

 bailing water, each washing will at length let fall a blue-grey 

 deposit. Precisely the same blue-grey powder is produced by 

 triturating black oxide of mercury with cold dilute nitric acid. 



