R. Bunsen on the Washing of Precipitates. 323 
Column I. shows the relation between the volume of the preci- 
pitate and that of the washing-water employed for each succes- 
sive decantation, column II. the number of decantations re- 
quired to diminish the amount of impurity to the necessary 
. extent, and column III. the total volume of water obtained 
om the several decantations. 
When the washing-process is performed in a beaker, the rela- 
tion between the volume of the precipitate and that of the li- 
quid may be easily determined by holding a strip of paper along 
the side of the vessel and marking upon it the respective 
the precipitate, the number of folds will give the argument in 
column I. to find in column II. the number of decantations 
| needed to wash to the required extent. If the washing be con- 
| ducted as in the ordinary method of filtration, funnels possess- 
| ing an angle of 60° must be invariably employed, and the capa- 
| cities of the various-sized filters once tor all determined by 
means of a burette. After the precipitate has been brought 
upon the filter and allowed to drain, it is mixed as thoroughly 
as possible with water from a graduated washing-flask. Call 
the amount of water thus necessary to fill the filter, and the 
Capacity of the empty filter &, then ar =* in column I; 
that is, the argument needed to find in column II the number 
of times it is necessary to refill the filter in order to wash the 
' Precipitate to the desired extent. 
y far prefer using this Table to employing the method 
generally followed of ascertaining the completion of the wash- 
Ing-process by evaporating a quantity of the filtrate on plati- 
num-foil, since in the latter case it is only possible to obtain an 
infallible proof when we have to deal with a precipitate possess- 
ing an extremely high degree of insolubility ; if the precipitate 
be soluble to any marked extent, the result is completely illu- 
as 
Ty. : 
In the process of filtration as hitherto conducted, the time 
tequired is so long and the quantity of wash-water needed so 
