Nature of White Precipitate. 227 



muriate of ammonia, containing 100 grains of the salt. The 

 change of colour in a few minutes was very evident. The so- 

 lution, when tested, contained coriosive sublimate. The boil- 

 ing was continued with four other portions of muriate of am- 

 monia, 100 grains each; when the calomel was entirely decom- 

 posed, 40 grains of mercury remained. Sixty grains of white 

 precipitate were obtained from the solutions by carbonate of 

 soda. There was no decomposition of the sal ammoniac. 

 With common salt I obtained the same results, mercury re- 

 maining, and white precipitate being thrown down from the 

 solutions, bj' liquid ammonia. Common salt is not so active 

 in producing these changes ; as ten portions of 100 grains each 

 were used before the decomposition of 100 grains of calomel 

 was complete. Muriate of potass and the earthy muriates 

 have, I have every reason to believe, the same power ; but I 

 did not push the experiments as in the case of soda and am- 

 monia." 



The action of chlorides upon calomel Mr. Hennell has pai-- 

 ticularly investigated in respect to common salt and muriate of 

 ammonia, those being the substances usually employed for the 

 purpose of washing calomel, under the idea of freeing it from 

 corrosive sublimate, an effect which they fulfil when employed 

 cold and in dilute solution only. But when perfectly pure 

 calomel is boiled for a few minutes in a solution of muriate of 

 ammonia or of common salt, and a portion of the liquor fil- 

 tered off and tested, a portion of sublimate is alwaj's found in 

 it ; and on boiling for a long time, the whole of the calomel is 

 decomposed, and compounds of sal ammoniac and corrosive 

 sublimate, and of common salt and corrosive sublimate, are 

 obtained, an equivalent portion of metallic mercury being at 

 the same time separated. 



These facts are peculiarly important in relation to the pre- 

 paration of calomel, inasmuch as the Pharmacopoeia directs 

 the use of a hot solution of muriate of ammonia, with the in- 

 tention of freeing it from any accidental admixture of corro- 

 sive sublimate ; and Dr. Henry, in describing the methods of 

 ascertaining the purity of calomel, directs it to be boiled in 

 solution of muriate of ammonia. *' When carbonate of po- 

 tassa," he observes, " is added to the filtered solution, no pre- 

 cipitation will ensue, if the calomel be pure*." Several other 

 chemists of eminence have given this as a criterion by which 

 to recognise the presence ot corrosive sublimate in calomel; 

 whereas it appears from Mr. Hennell's experiments, that the 

 protochloride of mercury is in such cases decomposed, and 

 that perchloridc is formed. 



•Elements of Expcrimcnlal (hcniistrv, 9th fdition, p. 588. 



F f 2 I laving 



