— 
R. Bunsen on the Washing of Precipitates. 321 
This analysis and ratio does not seem to agree with any of 
the hydrous silicates in Dana’s Mineralogy, and the species is 
therefore probably a new one. It occurs very sparingly im- 
planted upon Lesleyite, and like it, is most probably the product 
of the alteration of the corundum. 
I am indebted to Dr. Lea for specimens of all of these min- 
erals and for the use of his large specimens for comparison with 
each other. 
Cambridge, Mass., Jan. 8, 1869. 
Art, XXVIIL—On the Washing of Precipitates; by 
R. Bunsen.* 
_A precirrrate is washed either by filtration or by decanta- 
tion: in the former case the portion of liquid not mechanically 
retained is allowed to drain from the precipitate ; in the latter 
it is separated by simply pouring it away, the foreign substances 
contained in the precipitate being then removed by the repeated 
Upon the filtrate after filtration, V the volume of wash-water 
employed at each successive decantation, n the number of di 
cantations, and | the fraction expressing the proportion of the 
7 Mt % . . . * . . 
original amount of impurity still remaining in the precipitate 
after n decantations, then 
ee 1% 1 
Cet te ee OR 
=a) Se () 
Calling W the total volume of wash-water resulting from n 
decantations, then 
WVREW Gs ee es . (2) 
Wr 
(1 +0) = 
or W=nr( Va—1). Vahifee tert cn ss (3) 
If we differentiate W with respect tom and make the differ- 
* : 
Edin. and Dublin Phil. Mag., Jan. 1869. Translated from the 
i, p. 269. 
therefore 
a From Lond., blin 
Ann. der Chem. und Pharm , vol. exlviii, p. ‘ 
