296 Facts towards the 



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 filtered off 

 and tested, a portion of sublimate is always 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 prepara- 

 tion of calomel, inasmuch as the Pharmacopoeia directs the use of 

 a hot solution of muriate of ammonia, with the intention of freeing 

 it from any accidental admixture of corrosive 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 potassa," he observes, "is added to the filtered 

 solution, no precipitation will ensue, if the calomel be pure*." 

 Several other chemists of eminence have given this as a criterion 

 by which to recognise the presence of corrosive sublimate in calo- 

 mel ; whereas it appears from INIr. Hennell's experiments, that 

 the protochloride of mercury is in such cases decomposed, and 

 that perchloride is formed. 



The compounds of corrosive sublimate with other chlorides, 

 though noticed by many authors, and more particularly by Dr. 

 Davy, in his paper printed in the Philosophical Transactions for the 



boiling was continued with four other portions of niuriate of ammonia, 100 

 grains each ; when the calomel was entirely decomposed, 40 grains of mer- 

 cury remained. Sixty grains of white precipitate were obtained from the so- 

 lutions by carbonate of soda. There was no decomposition of the sal ammo- 

 niac. With "common salt I obtained the same results, mercury remaining, and 

 white precipitate being thrown down from the solutions, by 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 j but I did not push the experiments as in 

 the case of soda and ammonia." 



* Elements of Experimental Chemistry, 9th Edition, p. 588. 



