a nero Method of preparing ike PruJJlate ofPot-aJh, 3 1 



ftantial detail of the experiment of Meffrs. Meyer and Klaproth. I rcfolved, therefore, to 

 appeal to the decifion of experiments condudted with all pofllble accuracy and attention. 



The queftion in difpute is, whether or not a double eleftive affinity be exerted between 

 prufifiate of pot-alh and muriate of barytes ? If fuch an affinity really fubfifts, and produces, 

 on the admixture of the two folutions, two new compounds, one of thefe (the pruffiate of 

 barytes) muft neceflarily, from its known infolubility, appear under the form of a pre- 

 cipitate. Now, on mixing the two folutions, a precipitation generally takes place ; but 

 this, it is affirmed by certain chemifts, is owing to the impurity of the pruffiate of pot-afli, 

 and is occafioned by the decompofition of fulphate of pot-afh by muriate of barytes. The 

 truth of this explanation may be afcertained, either by an attentive examination of the pre- 

 cipitate, or by employing a pruffiate of pot-afh perfectly free from fuch impurity. 



The former of thefe lefts, as I have ftated in your Journal, II. 171, I employed feveral 

 months ago. Having found that prufTiated barytes is foluble in diluted muriatic acid, 

 •which exerts no adlion on the fulphate of that earth, I digefled a precipitate, thrown down 

 from muriated barytes by pruffiated pot-afh, with diluted muriatic acid. Part of the pre- 

 cipitate was taken up ; and the folution betrayed, to the proper tefts, decifive charaders of 

 its containing pruffiated barytes- Since the receipt of Mr. Kirwan's letter, I have repeated 

 this experiment feveral times ; occafionally with the refult, which has jufl now been flated ; 

 while, at other times, none of the precipitate was diffolved by the acid. Thefe con- 

 tradiiStory refults will be explained by what follows ; but not being able to devife the caufe 

 of them, at the time they occurred, I had recourfe to the remaining expedient, viz. the 

 employment of a pruffiate of pot-afh, perfedlly free from fulphuric falts. 



The difficulty of obtaining a pruffiate of the required purity in the common mode, is 

 hardly conceivable, except by thofe who have made the attempt. In your Journal, II. 171. 

 I have afTerted that both carbonate and fulphate of pot-affi are decompofed by pruffiated 

 barytes ; and it has fince occurred to me, that an excellent mode of preparing pruffiate of 

 pot-afh might be founded on thefe fa£ts. For the fmall portion of fulphate of pot-afh, 

 contained in all carbonates, may be expelled to be decompofed by the pruffiated barytes, 

 provided this laft be employed in fufficient quantity. On making the experiment, the re- 

 fult anfwered my expeftation ; and I fliall, therefore, give the procefs, which I have fince 

 frequently repeated with the fame fuccefs, under the form of a pra£tical rule. 



1. Calcine any quantity of carbonate of barytes in a flrong fire, a fufficient length of 

 time to expel its carbonic acid. Diflblve the pure barytes in boiling water, and add by 

 degrees pure Pruffian blue in powder, till it ceafes to be difcoloured. Filtre the folution 

 through paper, and if it fhould become muddy during cooling, owing to the depofit of oxyde 

 of -iron, filtre it again. After having flood a few hours, fmall yellowiffi cryftals will ap- 

 pear, which are the pruffiated barytes. From the remaining folution a further quantity 

 of cryftals may be obtained by evaporation. 



2. To a folution of carbonate of pot-afh, gently heated, add by degrees the pruffiate of 

 barytes in powder^ till the folution no longer reftores the colour of reddened litmus paper. 



Rather 



