On a Quantitative Blowpipe Assay of Chromium. 393 



down^ the vessel is taken tip ^tli tlie pincers and the fused bi- 

 sulphate made to spread itself on the sides as much as possible. 

 Since no heat must be applied in the solution of the mass, it 

 would be extremely tedious to wait for the solution of the drj 

 bisulphate thus remaining. Five to six drops of oil of vitriol 

 are therefore added, and the mass reheated wdth these; on cool- 

 ing subsequently, it remains sufficiently soft to dissolve in water 

 almost instantaneously. The cover is first rinsed into a small 

 beaker, which maybe filled to about one-third; the whole of 

 this fluid is afterwards poured into the platinum vessel, and 

 the solution promoted by stirring, so as to prevent any parts of 

 the mass getting sensibly heated. When many foreign sub- 



I stances are present, it is best to add some more acid subse- 



quently. 



The precipitate cannot be washed w'ith pure water. Salts, es- 

 pecially AmCl, prevent its diflfusion in water, yet not sufficiently 

 to make it wash expeditiously. It is necessary to involve it in 

 some other precipitate settling quickl}', not interfering with the 

 separations, and volatile in the subsequent ignition. Such a one 

 I found in the chlorosulphid of mercury. Half a spoonful of 

 corrosive sublimate is added to the bisulphate solution, and 

 while stirring violently, dilute sulphid of ammonium is added 

 until, w^hen the precipitate turns of a dark olive tint, the liquid 

 above the flocculi is perceived to be clear. It may then at once 

 he filtered; during the washing a little HgCl is thrown on the 

 filter. Two and a half inch filters are best; the amount of 

 ashes they give must of course be accurately known. 



The filter is dried between blotting sheets, formed into a square 

 package and incinerated in the platinum torrefaction dish, in a 

 roasting furnace, after having driven off most of the mercury 

 over the spirit lamp. The incineration is somewhat difficult; 

 after it is complete, the loose mass is transferred to the platinum 

 crucible previously counterpoised on the balance together with 

 a small wire tripod (fig. 3). The crucible being ^ 



covered with the torrefaction dish, is placed on the 

 furnace, which is closed and provided with char- 

 coal cover. A strong and steady heat is then 

 Maintained for four to six minutes, making the 

 flame dart out at the chimney -hole. The mass will 

 then appear dark green all over; if the heat has 

 been strong, some chroniate may have formed, 

 Tvdiich w^ouTd vitiate the result. The w^holc is therefore moist- 

 ened with a drop of strong sulphid of ammonium, then carefully 

 dried over the lamp and gently re-ignited. Deducting the filter 

 ashes from the w^eight of the residue, the result is calculated, 

 100 of residue corresponding to 47 '3 of chromic oxyd. 



SECOND SERIES, VOL. XXIV, NO. 71.— SEPT., 185t. 



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