﻿4 Prof. R. Bimsen on the Washing of Precipitates. 



I by far prefer using this Table to employing the method ge- 

 nerally followed of ascertaining the completion of the washing- 

 process by evaporating a quantity of the nitrate on platinum-foil, 

 since in the latter case it is only possible to obtain an infallible 

 proof when we have to deal with a precipitate possessing an ex- 

 tremely high degree of insolubility ; if the precipitate be soluble 

 to any marked extent, the result is completely illusory. 



In the process of filtration as hitherto conducted, the time 

 required is so long and the quantity of wash- w r ater needed so 

 great that some simplification of this continually recurring ope- 

 ration is in the highest degree desirable. The following method, 

 which depends, not upon the removal of the impurity by simple 

 attenuation, but upon its displacement by forcing the wash- 

 water through the precipitate, appears to me to combine all the 

 requisite conditions and therefore to satisfy the need. 



The rapidity with which a liquid filters depends, cater is paribus, 

 upon the difference which exists between the pressure upon its 

 upper and lower surfaces. Supposing the filter to consist of a 

 solid substance, the pores of which suffer no alteration by pres- 

 sure or by any other influence, then the volume of liquid filtered 

 in the unit of time is nearly proportional to the difference in 

 pressure : this is clearly shown by the following experiments, 

 made with pure water and a filter consisting of a thin plate of 

 artificial pumice-stone. The thin plate of pumice was hermeti- 

 cally fastened into a funnel consisting of a graduated cylindrical 

 glass vessel, the lower end of which was connected with a large 

 thick flask by means of a tightly fitting caoutchouc cork. The 

 pressure in the flask was then reduced by rarefying the air by 

 means of a method to be described upon another occasion ; and 

 for each difference of pressure p, measured by a mercury column, 

 the number of seconds t was observed which a given quantity of 

 water occupied in passing through the filter. The following are 

 the results : — 



I. 



metre. 



i. 



pi. 



0-179 



91-7 



16-4 



0-190 



81-0 



1-5-4 



0-282 



52-9 



14-9 



0-472 



330 



15-6 



In the ordinary process of filtration, p on the average amounts 

 to no more than 0*004 to 0'008 metre. The advantage gained, 

 therefore, is easily perceived when we can succeed by some 

 simple practicable and easily attainable method in multiplying 

 this difference in pressure one or two hundred times, or, say, to 

 an entire atmosphere, without running any risk of breaking the 



