﻿Prof. R. Bun sen on the Washing of Precipitates. 15 



unfrequently happens, even in the hands of experienced mani- 

 pulators, in consequence of the agitation it is necessary to give 

 to the contents of the filter to effect their complete washing, 

 that the surface of the filter becomes injured and torn, so that 

 the precipitate becomes mixed with filaments of paper ; this is 

 particularly the case in using hot water. Supposing the preci- 

 pitate to consist of mixed hydrates of the sesquioxides (for ex- 

 ample, iron and alumina), it will be found, on redissolving in an 

 acid, that the filaments, like tartaric acid, prevent the complete 

 separation of these substances by subsequent precipitation ; thus 

 the alumina will contain iron, and on precipitation by means of 

 ammonium sulphide will be coloured black. On the other 

 hand, by employing the new method the precipitate coheres so 

 firmly that the introduction of this source of error is impossible, 

 even by using common grey filter-paper. The most gelatinous 

 precipitates, as hydrated ferric oxide, alumina, &c, adhere to the 

 filter in a thin coherent layer, and may be removed, piece after 

 piece, so completely that the paper remains perfectly clean and 

 white. The advantage thus gained where it is necessary to 

 transfer mixed precipitates to another vessel in order to effect 

 their subsequent separation is evident. 



The filter-pump, moreover, is exceedingly serviceable in sepa- 

 rating precipitates or crystals from syrupy mother-liquors. Thus 

 honey-sugar may be so completely separated from the thick 

 viscid liquid in which it forms by a filter of coarse grey paper, 

 that it remains only slightly coloured, and by a single crystalli- 

 zation from alcohol may be obtained in small white shining 

 needles. And since the bulk of the moist precipitates, particu- 

 larly that of the more gelatinous, is so much diminished under 

 the high pressure, the precipitate only occupying one-third to 

 one-sixth of its bulk under ordinary circumstances, a filter of 

 one-third to one-sixth of the size usually employed may be 

 taken, and thus the amount of ash proportionately lessened. 



As the water air-pump suffers no injury from the presence of 

 corrosive vapours or gases, we can equally well employ it to filter 

 liquids containing nitrous acid, sulphurous acid, fuming nitric 

 acid, chlorine, bromine, volatile chlorides, &c. In such cases I 

 use a peculiar filtering arrangement, consisting of a cylindrical 

 glass vessel, the lower end of which is drawn out before the 

 blowpipe to the form shown in fig. 5 ; in this drawn-out portion 

 a thin plate, 1 or 2 millims. in thickness, of artificial pumice, 

 such as is used by polishers, is packed water-tight by means of 

 asbestos. This apparatus is arranged for the purpose required 

 exactly as the funnel in the method of filtration by pressure 

 above described. In order to have a number of these filters in 

 readiness, a pumice-stone cylinder of the required diameter is 



