﻿Prof. R. Bunsen on the Washing of Precipitates. 9 



longer than that needed to pour a liquid slowly from one vessel to 

 another. In filtering by means of paper, the precipitate gradually 

 closes up the pores of the filter, and accordingly such an ex- 

 traordinary acceleration as this can no longer be expected. But 

 the following examples will show the saving of time and labour 

 the method effects, even under all unfavourable conditions. For 

 these experiments I have purposely chosen the hydrated chro- 

 mium sesquioxide, since it is one of the most difficult of preci- 

 pitates to wash thoroughly. A solution of chromium chloride 

 was prepared by acting with fuming hydrochloric acid upon 

 potassium dichromate; and by means of a measuring -vessel, which 

 allowed the amount of chromium to be estimated to within 

 0*0001 grm., successive portions of the liquid were withdrawn, 

 and the chromium oxide contained in them precipitated with the 

 usual precautions by ammonia. The volume of liquid, the 

 quantity of ammonia employed, the time occupied in boiling and 

 in permitting the precipitate to settle, the angle of inclination 

 possessed by the funnel, and the size of the filter were the 

 same in all the experiments. All the precipitates were washed 

 with hot water, and, after burning the filter, ignited over the 

 blowpipe for a few minutes ; in weighing, the platinum cru- 

 cible was tared by one of about equal weight, and the posi- 

 tion of equilibrium of the beam determined by vibrations. 



I first attempted to filter one of the precipitates in the or- 



V 



dinary way. — amounted to 2 ; and consequently, from the 

 v 



Table, 8'4 fresh additions of water were required in order to 



wash the precipitate to the j-ooo~o part. The times required 



were as follows : — 



In transferring the precipitate from the! ' 



beaker and allowing it to drain . . J 



For the first addition of water to run through. 48 



„ second „ „ 70 



« third „ „ _80 



Total length of time .... 238 



became 

 cause. 



At this point the experiment was discontinued, as the filtrate 



came turbid. A second experiment failed from the same 



use. 



Accordingly I attempted to wash the precipitate by decanta- 

 tion. The volume of the precipitate amounted to about 30 cub. 

 centims.: the quantity of water required to fill the beaker was seven 



times the volume of the precipitate j hence - was 7, and the 



