\Q4< ON THE NATURE OF PRECIPITATES. 



by double affi- tion to a change of bafe of the two falts by double affinity. I 

 mt y* then purpofed to examine whether this might not be abfolutely 



the faipe phenomenon which had given rife to the fufpicionof 

 the metallic nature of barites. 

 Mr. Henry's I was anticipated in thefe refearches by Mr. W, Henry. 



tYom^e^Pnilof. ** G * ias mccee ded in placing this point of theory in the cleareft 

 Journal, light: I fhall repeat in a few words the experiments tha| 



him to it, and which he publifhed in Mr. Nicholfon's Journal"*; 

 after which I lhall make fome reflections upon one of the. con- 

 ferences he deduced from them, which appears to me to re- 

 quire amendment. . 

 Pruffiate of ba- Mr. Henry's firft objeft was the dirept compofition of pruf- 

 utes formed, ^ ale f barites, in order to obtain a perfectly pure pruffiate of 

 potaih: the following is his procefs, which he affirms thai lie 

 carried into effect, and was fuggefted tq him by obferving the 

 decomposition of the falphate and carbonate ofpqtafliby iho 

 pruffiate of barites. 

 by calcining the He calcines the carbonate of barites ; he diflfolves this earth 

 ^°f a ^( v ^- in boiling water, and adds Pruffian blue till it is no longer dif- 

 the earth in coloured. The filtered liquor fometimes becomes turbid by 

 p at< <? ^i add * cooan a> aHC * depofes a little oxide of iron. He filters it again, 

 till it is no Ion- and at the end of feveral hours fmall yellowim cryftals are 

 ger difcoloured formed, which are the pruffiate of barites. He obtains an 



by the earth ,...', . . r 



feizing the p. additional quantity by evaporation. 



acid. This pruffiate of barites in powder is thrown into a hot fo- 



pruffiate of b a- ^ on of carbonate of potaih, until it no longer reflores tha, 

 lites is decom- colour of blue paper. The author obferves, that it is better to. 

 P °t f ^ C t r fh°~ e . m V^°y more pruffiate than is neceflary for the decomposition- 

 and thus gives of the carbonate. He digefts the mixture for about an hour ; 

 fine cryftals of t h en filters the liquor, evaporates it flowly, and obtains by 

 a fh. this method very fine cryftals of pruffiate of potam. 



Laft purification Thefe cryftals neverlhelefs fometimes contain as much a* 

 by acetous acid, q.24 of oxide of iron ; the grcateft, part of this is feparated by 

 digefting the liquor before filtrated, with a little, acetic acid, 

 which has the advantage of forming with the polalh an un^ 

 cryftallizable fait, and confequently cannot mix with the cry- 

 ftals of the pruffiate. 



The precipitated carboh&e' of barites may again be u.'e D ' 



the fame operation. 



... 



* Vol. IV. QWhcrJ pi.^O, 170. 



•Let 



