On some of the Combinations of Mercury. 49 



water is poured upon either of these salts a precipitate falls 

 down which is of a yellow colour, and the solution deposits 

 chrystals which are in the form of rhombs. If, instead of boil- 

 ing water, cold water is poured upon the same chrystals, the 

 precipitate which falls down is white. 



It appears, therefore, that protoxide of mercury forms with 

 nitric acid three chryslalizable compounds which may be named 

 from their form, hexagonal, acicular, and rhombic pronitrates 

 of mercury. Besides these, the protoxide may be united in 

 other proportions with the acid. Thus by triturating 92 parts 

 of mercury and 100 of peroxide with 50 of acid, adding so much 

 water as is necessary to give the materials the consistence of a 

 paste, a compound is formed of a white colour. The acid may 

 be made to take up even a larger quantity of metallic oxide, but 

 in this case more labour is necessary. The theory of the action 

 is that the peroxide imparts one proportion of oxygen to the 

 metal, both being converted into protoxide, which unites with 

 the acid. The compound formed by using-- the above propor- 

 tions is probably the same as the rhombic pronitrate, and as 

 the precipitate which falls down when cold water is added to 

 the hexagonal or acicular pronitrate, and probably between this 

 compound and the yellow precipitate formerly mentioned, com- 

 bination takes phce in every proportion. 



The hexagonal pronitrate is most readily obtained by dissolv- 

 ing mercury in diluted ncid in the manner formerly described, 

 the process being stopped before the whole of the acid is taken 

 up, or, if continued, till no acid remains, by adding a small 

 quantity of acid to the solution previously to cooling it. The 

 solution of it in water when hot bears dilution to a considerable 

 extent without decomposition, but when the chrystals are acted 

 upon either by hot or cold water they are decomposed. The 

 chrystalized salt remains permanent, in the air. 



The acicular pronitrate may be obtained in the same manner 

 as the hexagonal, continuing the process till all the acid is 

 taken up. The solution bears dilution in the same manner as 

 the solution of the hexagonal pronitrate, but not to the same 

 extent. The dry chrystals when exposed to the air become 

 yellow, a change probably resulting from the formation of hex- 

 agonal pronitrate on the one hand and of the yellow insoluble 

 salt on the other. The presence of a very slight excess of acid 

 prevents this change, the chrystals remaining permanent. 



These two salts are convertible, the one with the other, ac- 

 cording to circumstances. When a saturated solution of pro- 

 nitrate contains oniy a slight excess of acid, it sometimes 

 deposits acicular chrystals, after which the acid which remains 

 in the solution, acting upon the chrystals, combines with them, 



