Jl54' MTilt PhlJJIntt if Irjn. 



cipitate, wliich is an oxide, incapable of ittraaing any thing from the air of the atmofphcre, 



nor from the oxygenated muriatic acid. 



This oxiJc, according to the experiments of Mr. Prouft, contains A'o °f oxygenc, and 

 is capable of forming the baGs of a ferics of combinations, which bear the fame relation to 

 thofe which afTorJ the black oxide, as the red fulphate of iron bears to the green fulphate. 



So that in whatever degree of alteration an ordinary fulphate may be examined, it will 

 be, found- lo coiifill merely of thefe two falts, mixed in different proportions. 



^Vhence it foUoiv.s, that a fulphuric or muriatic folution of this metallic fubftance is 

 nothing but a mixture of the two fails, one of which has for its bafis the oxide of iron, 

 containing T^j'^ of .oxygcnc, and the other the fame oxide, containing //„- of oxygene. 

 And as little attention has hitherto been paid to the diflinclivc properties of thefe falts j 

 efieds have frequently been attributed to the one, which belong exclufively to the other. 



We mud therefore dillinguifli two fulphates, two muriates, two arfeniates, two pruf- 

 fiates, &c. : the latter faits more particularly conlUtute tlie objed of Mr. Prouil's Memoir. 



The White Pniffwte. 

 IX order to obtain this fait, a folution of the perfectly pure green fulphate muft be taken 

 and mixed with a well faturated folution of the prulTiate of pot-afli. This laft folution, if 

 perfeft, affords cryflals of a beautiful orange-yellow in tetrahedral pyramids, truncated near 

 their bafe. 



The green fulphate of iron is obtained and preferred in, a ftatc of purity, by keeping it 

 in a well clofcd vefTel quite full, in which a fmall piece of iron or tin is put. The fame 

 purpofe is anfvvered by reducing the red oxide, which may exift in the folution, to the ftate 

 of black oxide, by a mixture of water charged with fulphuratcd hydrogene gas. This ful- 

 phate, after purification, ought not to change by the addition of the gallic acid. In 

 this ftate, its colour is an extremely weak (liade of fea-green. The green colour does not 

 appear, unlefs the folution be in contact with the atmofpheric air. 



After having poured the folution of the prufTiate upon the fulphate of iron, tlie bottle mufl 

 be immediately clofed. A plentiful white precipitate immediately falls down, which foon 

 afterwards affumes a green tint, occafioned by the air contained in the bottle, and likewife 

 by another caufe. But if the vefTel remain clofed this fliade does not become deeper, and 

 light alone does not produce any change. 



It is advifable to pour an excefs of the pruffiate upon the fulphate, in order to com- 

 plete the decompofition. After feveral hours rej:ofe, this pale pruffiate is covered with a 

 yellow liquor, which is a mixture of tire pruffiate and the fulphate of pot-afh. This liquor 

 retains in folution a fmall quantity of white pruffiate, which acquires a blue colour by ab- 

 forption of oxygene, when it is placed in contact witli the atmofpheric arr. By this union, it 

 becomes infolublc, and falls upon that which is at the bottom. The laft-mentioned pre- 

 cipitate receiving in its turn the imprellion of the atmofphere, foon becomes blue at its fur- 

 face. The intenfity of colour fuccelTively increafes, and proceeds downwards, until at 

 length the white pruffiate becomes converted into PruHian blue. 



The a£lion of the oxygene of the atmofphere, in thefe circumflances, is ftill more con- 

 firmed by the phenomena which are fcen when the white pruffiate is expofed to the a£lion 



of 

 4 



