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



work, in comparison with the method generally adopted, is con- 

 siderably shortened and simplified. 



From the above experiments it appears that the time neces- 

 sary to filter and dry a quantity of chromium sesquioxide, 

 hitherto requiring about 7 hours, is reduced by the new method 

 to 13 minutes. This saving of time is, moreover, proportion- 

 ately greater in the case of precipitates more easily filtered than 

 hydrated chromium sesquioxide. Particularly is this so in se- 

 parating a finely suspended precipitate from a large volume of 

 water. Under these circumstances the clear fluid runs through 

 the filter in a continuous stream, so rapidly that it is scarcely 

 possible to maintain the supply ; the entire operation, in fact, 

 requires scarcely more time than that necessary to pour a liquid 

 from one vessel to another. Filtration, therefore, may be effected 

 as quickly through the smallest as through the largest filter. 

 Moreover the exceedingly small amount of water required to 

 wash a precipitate completely renders unnecessary the tedious 

 evaporations which by the older method are almost inevitable 

 when the filtrate is needed for a further separation. Thus the 

 introduction of impurities from the action of the liquid upon the 

 dish in the course of evaporation is prevented ; and also the loss 

 due to the slight solubility of the greater number of precipitates 

 in the wash-water is reduced to a minimum. Supposing we 

 had to analyze an alkaline chromate in which the quantity of 

 chromic acid is equivalent to 0-2436 grm. chromic sesquioxide, 

 as in the above described experiments, then to determine the 

 proportion of alkali we should, by using the older method, re- 

 quire the preliminary evaporation of about 1050 cub. centims. of 

 liquid; by the new method the evaporation of 40 cub. centims. 

 only is necessary.. Now by employing the best form of water- 

 bath, i. e. one possessing a constant water-level, such as is used 

 in my laboratory, it is possible, under favourable circumstances, 

 to evaporate in a porcelain dish 1 cub. centim. of water in 27 se- 

 conds. Consequently the evaporation of the filtrate obtained by 

 the older method would occupy about 8 hours, whilst by the 

 new 18 minutes only are required. The total length of time 

 needed to filter the chromium sesquioxide, wash and dry the 

 precipitate, and evaporate the filtrate is reduced, therefore, from 

 14 or 15 hours to about 32 minutes. 



The experience I have subsequently gained in my laboratory, 

 where the method has been in general use for the last nine 

 months, fully confirms the above results. It has shown that, on 

 the average, three or four analyses can now be made in the time 

 formerly demanded by a single one. 



Another and an inestimable advantage springs from the pecu- 

 liar condition of a precipitate filtered by this method. It not 



