On a Quantitative Blowpipe Assay of Chromium. 



391 



potasli-ley. It is however onlj an indefinite quotum of the 

 chrome alum in solution w^ich is thus precipitated. If the so-" 

 lution be still further heated, until oil of vitriol volatilizes, the 

 whole of the chromium is precipitated in the form of a salt of 

 the same composition as above, but which is slowly resolved into 

 KO, SO 3 and CrsOs, SSOs (insoluble) by hot water alone, while 

 it is not changed by cold water. When ignited, sulphuric acid, 

 escapes and IvO, S03+Cr203 are left behind. It is upon the 

 formation of this salt that I have based a general method of de- 

 termination; finding that its formation is quite independent of 

 the presence of any acid, and many basic substances, so long as 

 an excess of SOn and potash are present at the same time. 

 The solution containing the potash salts requires to be evapo- 



rated. This evaporation would be most safely and expeditiously 



1. (Half-size). 



performed in a long-necked glass flask, as with other voluminous 

 filtrates; but it is difficult to find glass which will safely resist 

 the subsequent bisulphate fusion. Even the porcelain dishes do 

 not resist many of the latter. I have therefore contrived for the 

 purpose a little evaporator of which fig. 1 is 

 a half size representation, and which is very 

 useful in many other cases. The lower part, 

 a, is of platinum, the upper, &, of thin glass ; 

 it is set on the lower vessel where the curva- 

 ture of the rim begins. The seam between 

 the two parts is effectually closed up by capil- 

 lary attraction, as soon as any fluid is boiled 

 in tbe vessel; so that the whole represents a 

 close flask, which may be filled with fluid to 

 within a few millimetres of the seam without 

 ^'ny fear of loss, even if, in boiling, the froth 

 should rise into the neck of the glass cover. 

 Care must of course be taken not to heat the 

 nm so as to cause boiling of the closing fluid. 

 Silicic and manganic acids, when present in 



a 



the solution, must be precipitated he/ore filter- 

 ing; the latter by a few drops of alcohol, and 

 the former by a spoonful of nitrate of ammonia. In this case 

 the solution must be boiled before filtration, to make the SiOa 



collect in flakes, otherwise there is difficulty in filtering. The 

 crucible having cooled after the nitre fusion, is therefore transfer- 

 red, together with its cover, to tbe evaporator, which is about 

 one-third full of water. The solution may then be accelerated 

 ty heating, after which the precipitants just named are added 

 ^d the -VN^ole boiled for half a minute, the vessel being covered 

 ^ith the watch-o-lass-shaped platinum cover used in the bisul- 

 P^^ate fusion (see^below); after this the fluid is transferred to one 

 of the small beakers, vessel and cover being economically rinsed 



