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



by the water-stream in my laboratory, in which six of these pumps 

 are used, amounts to about 7 millims. in winter and 10 millims. 

 in summer. The filtration is made in the following manner : — 

 The flask standing in the metallic or porcelain vessel (fig. 3) is 

 connected by means of the slightly drawn-out tube k with the 

 caoutchouc tube h attached to the pump, the cock a having been 

 previously opened and the properly fitted moistened filter filled 

 with the liquid to be filtered. As usual, the clear supernatant 

 fluid is first poured upon the filter ; in a moment or two the fil- 

 trate runs through in a continuous stream, often so rapidly that 

 one must hasten to keep up the supply of liquid, since it is ad- 

 visable to maintain the filter as full as possible. After the pre- 

 cipitate has been entirely transferred, the filtrate passes through 

 drop by drop, and the manometer not unfrequently now shows a 

 pressure of an extra atmosphere. The filter may be filled (in 

 fact this is to be recommended) with the precipitate to within a 

 millimetre of its edge, since the precipitate, in consequence of the 

 high pressure to which it is subjected, becomes squeezed into a 

 thin layer broken up by innumerable fissures. As soon as the 

 liquid has passed through and the first traces of this breaking up 

 become evident, the precipitate will be found to have been so 

 firmly pressed upon the paper, that on cautiously pouring water 

 over it it remains completely undisturbed. The washing is 

 effected by carefully pouring water down the side of the funnel 

 to within a centimetre above the rim of the filter : the washing- 

 flask for this purpose is not applicable ; the water must be poured 

 from an open vessel. After the filter has in this manner been 

 replenished four times with water and allowed to drain for a few 

 minutes, it will be found to be already so far dried, in conse- 

 quence of the high pressure to which it has been subjected, that 

 without any further desiccation it may be withdrawn, together 

 with the precipitate, from the funnel, and immediately ignited, 

 with the precautions to be presently given, in the crucible. 



If the porosity of a paper filter containing a precipitate were 

 as unalterable as that of a pumice-stone filter, the experiments 

 above described would show that the times required for filtration, 

 according to the old method on the one band, and the new one 

 on the other, would be inversely proportional to the difference in 

 pressure in each case; that is, by using the pump under the full 

 pressure of about 740 millims., the time needed to wash a pre- 

 cipitate, occupying by the old process an hour, would at the 

 utmost not amount'to more than 30 seconds. In using these 

 pumice filters (about which I will speak presently) to drain 

 crystals from adhering mother-liquors, or, say, to wash crystals 

 of chromic acid by means of concentrated sulphuric acid and 

 fuming nitric acid, the time occupied in the filtration is scarcely 



