On the Purity of Drugs and Medicine. gg 



fupernatant liquor was decanted immediately, the precipitate, after having been waflied, 

 proved to be fimply fulphate of barytes ; but when the two folutions were allowed'to ftand 

 together fome time, before feparatlng them from the precipitate, this laft had conftantly the 

 properties of a mixture of fulphate of barytes, with prufliate of barytes ; and fometimes cryf- 

 tals of the latter fait were perceivable in the mixture. 



Before taking my leave of the fubjedt, allow me to corre£l: an erroneous flatement, which 

 I have made in your Journal, II. 171. I have there ftated that cauftic barytes does not pre- 

 cipitate prufllate of pot-afli. This experiment I have fince repeated with pure prufliate de- 

 fcribed above. No immediate precipitation enfued (the circumftance that before mifled 

 me) ; but after a few hours fmall cryftals were depofited, which had every character of 

 prufliate of barytes. It follows, therefore, that in a table of the eleftive afhnities of PrulEan 

 acid, barytes fliould be placed above pot-afti. 



I am. Sir, very refpe£lfully, 



your obedient fervant, 



WILLIAM HENRY. 



VIII. 



On the Genuinenefs and Purity of Drugs and Medicines. By Mr. Fred. Accum. 

 (Continued from page 122. Vol. ILJ 

 Examination of Alkaline Substances. 

 Prepared Kali. Pharm. Lond. Salt of Tartar. 



T. 



HE fait of tartar of the fliops, if prepared according to the London Pharmacopeia, can- 

 not be obtained pure from pearl-aflies by mere folutions, colaturc, evaporation, and cryf- 

 tallization •, hence this article generally contains a confiderable portion oi fulphate and muriate 

 ofpotafj, and is never perfedlly free {xomftliceous and calcareous earth. In order to difcover 

 its purity, one part ought to be diflbived in eight parts of diftilled water, and then neutra- 

 lized with pure nitric acid : if the mixture remain perfeftly tranfparent when cold, the 

 alkali is free from filiceous earth ; whereas on the contrary, if filiceous earth be prefent 

 the mixture will become turbid, the filiceous earth will be feparated, and its quantity may 

 thus be afcertalned. 



Sulphate of pot-afh, the next article generally found in this fait, is beft difcovered by 

 adding gradually to a fimilar neutralifed folution, a folution of muriate of barytes, or acetite 

 of lead." If fulphuric acid be prefent, a white precipitate will be produced, but no fuch eiFedl 

 will take place if no fulphuric combination be prefent. One hundred parts of the precipi- 

 tate, occafioned by the admixture of the muriate of barytes, indicate 26 per cent, of ful- 



VoL. IV.— April 1800. F phuric 



