﻿Prof. R. Bunsen on the Washing of Precipitates. Vd 



the crucible. Hitherto, therefore, it has been necessary to dry 

 the filter and precipitate before ignition. Now to dry a quantity 

 of hydrated chromium sesquioxide containing 0*2436 grin. 

 Cr 2 O 3 in a water-bath at 100° C. requires at least five hours ; and, 

 moreover, bringing the dried precipitate into the crucible, burn- 

 ing the filter, and gradually igniting the mass is in the highest 

 degree tedious and troublesome. All this expenditure of time 

 and labour may be saved by employing the new method. By 

 its means a precipitate is as completely dried upon the filter in 

 from 1 to 5 minutes as if it had been exposed from 5 to 8 hours 

 in a drying-chamber; and it can immediately, filter and all, 

 be thrown into a platinum or porcelain crucible and ignited 

 without the slightest fear of its spurting. By operating in the 

 following manner the filter burns quietly without name or 

 smoke ; this phenomenon, although remarkable, easily admits of 

 an explanation. The portion of filter-paper free from precipi- 

 tate is tightly wrapped round the remainder of the filter in such 

 a manner that the precipitate is enveloped in from four to six 

 folds of clean paper. The whole is then dropped into the pla- 

 tinum or porcelain crucible lying obliquely upon a triangle over 

 the lamp, and pushed down against its sides with the finger. 

 The cover is then supported against the mouth of the crucible in 

 the ordinary way, and the ignition commenced by heating the 

 portion of the crucible in contact with the cover. When the 

 flame has the proper size and position, the filter carbonizes 

 quietly without any appearance of flame or considerable amount 

 of smoke. When the carbonization proceeds too slowly, the 

 flame is moved a little towards the bottom of the crucible. After 

 some time the precipitate appears to be surrounded only by an 

 extremely thin envelope of carbon, possessing exactly the form 

 (of course diminished in size) of the original filter ; the flame is 

 then increased, and the crucible maintained at a bright-red heat 

 until the carbon contained in this envelope is consumed. The 

 combustion proceeds so quietly that the resulting ash surround- 

 ing the precipitate possesses, even to the smallest fold, the exact 

 form of the original filter. If the ash shows here and there a 

 dark colour, it is simply necessary to heat the crucible over the 

 blowpipe for a few minutes to effect the complete removal of the 

 trace of carbon. This method of burning a filter is extremely 

 convenient and accurate ; it is only necessary to give a little 

 attention at first to the slow carbonization of the paper, after 

 which the further progress of the operation may be left to itself. 

 Gelatinous, finely divided, granular, and crystalline precipi- 

 tates, such as alumina, calcium oxalate, barium sulphate, silica, 

 magnesium ammonium phosphate, &c v may with equal facility 

 be treated in this manner ; so that even in this particular the 



