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6. Evaporate the filtred liquor to about one quarter, and fet it to cryffallize; after a few days, yellowifh, cubic, or quadrangular cryftals will be found mixed with fome tartar vitriolate, and calx of iron ; pick out the yellowilh cryftals, lay them on blotting paper, and rediflolve them in four times their weight of cold water, to exclude the' tartar vitriolate. 7. Eflay a few drops of this folution with ba- rytic lime-water, and add fome tothe remainder; if neceffary, filtre off the folution, and fet it to cryftallize for a few days; thus the barytic lime, if any fhould remain, will be precipitated ; if the cryflals, now obtained, are of a pale yellow co- lour, and difcover no bluifh ftripes when fprin- kled over with marine acid, they are fit for ule; but, if they flill difcover bluifh or grcen ftreaks, the folutions and cryflallizatians inuft be re- peated. 8. Thefe cryftals muH: be kcpt in a well- ftoppcd bottle; and I think it right to fill it with fpirit of wine, to prelerve them from the air, as they are infoluble in fpirits. 9, Before they are ufed, the quantity of iron they retain (and which I therefore call their re- tent) fhould be afccrtained, by heating 100 grains to rednefs for half an hour in an open crucible, the Pruflian acid will bc confumed, and the iron will remain in the flate of a reddifh brown mag- fietic calx, which fhould be weighed,. and noted ; if the calx be black, it will weigh conllderably more thanit fhould, and muft be calcined anew; the red calx is half the weight of the Pruflian blue, aflorded by the Pruflian alkali; its weight muftbe fubtrafted from that of metalbc precipi- tates ibnncd by this teil, lience the weight of