﻿Prof. R. Bunsen on the Washing of Precipitates. 17 



comes somewhat rarefied • and in order to employ the consequent 

 difference in pressure (amounting to a column of mercury about 

 0*2 metre in height) for the purpose of nitration, it is only ne- 

 cessary to connect the mouth of the upper bottle with the tube 

 of the filter-flask. When the water has ceased to flow, the po- 

 sition of the bottle is reversed, when the operation recommences. 

 So small a pressure as 0*2 metre suffices to render the filter and 

 its contents so far dry that they may be immediately withdrawn 

 from the funnel and ignited without any other preliminary de- 

 siccation. The following experiment, made with a portion of the 

 same solution of chromium used in the former determinations, 

 will serve to show the saving of time effected by this simple ar- 

 rangement : — 



XL 



Transferring the precipitate with 14 cub. \ -.}, 

 centims. of water J 



For a single addition of 26 cub. centims. ~\ ~ 

 of wash-water to run through . . J 



To drain the precipitate 4 



Time required in washing ... 25 



Weight of the precipitate . . 0*2435 grm. 

 Volume of wash-water . . 40 cub. centims, 

 Pressure in manometer . . 0*184 metre. 



This amount of chromium sesquioxide (0*2435 grm.) differs from 

 the mean of the former experiments (0*2436 grm.) by one-tenth"of 

 a milligramme only, and shows that even by a pressure of 0*184 

 metre the Washing is as complete by the single addition of 26 

 cub. centims. of water. The duration of the filtering process in 

 the former experiments ranged from 12 to 14 minutes under a 

 difference of pressure amounting to from 0*53 to 0*572 metre ; 

 in the last experiment it required 25 minutes under a pressure 

 of 0*184 metre, or about double the length of time. The time 

 needed to analyze potassium chromate in the former case was 

 reduced from 14 hours to 32 minutes ; by the latter method the 

 reduction would be from 14 hours to 44 minutes. 



The employment of the second method is particularly to be 

 recommended to beginners in qualitative analysis. The experi- 

 menter needs only a single funnel, he is obliged to work care- 

 fully and cleanly, and the great saving of time and work amply 

 compensates for the little trouble needed to reverse from time to 

 time the position of the bottles. 



I believe that the above-described water air-pump will soon 

 become an indispensable piece of apparatus in chemical labora- 

 tories. It not only serves as the most convenient method of 



Phil. Mag. S. 4. Vol. 37. No. 246. Jan. 1869. C 



