CHEMISTRY. 211 



a pressure of 0.472 metre the same quantity ran through in 33.0''. 

 In the ordinary process of filtration the difference in pressure 

 amounts to scarcely more than 0.005 metre. The advantage 

 gained, therefore, is easily perceived when we can succeed by 

 some simple, practicable, and easily attainable method in multi- 

 plying this difference in pressure one or two hundred times, or, 

 say, to an entire atmosphere, without running any risk of breaking 

 the filter. The solution of this problem is very easy; a glass 

 funnel is chosen possessing an angle of as near 60° as possible, 

 the walls of which must be perfectly free from inequalities of 

 every description, and into it is placed a second funnel made of 

 extremely thin platinum-foil, the sides of which possess exactly 

 the same inclination as those of the glass funnel. An ordinary 

 paper filter is then introduced into this compound funnel in the 

 usual manner; when carefully moistened and so adjusted that no 

 air-bubbles are visible between it and the glass, this filter, vvhon 

 filled with a liquid, will support the pressure of an extra atmos- 

 phere without breaking. In order that the additional pressure of 

 an atmosphere may be produced the filtered liquid is received in 

 a strono^ cr]ass flask instead of in a beaker. This flask is closed 

 by a doubly perforated caoutchouc cork, through one of the holes 

 of which the neck of the glass funnel is passed ; through the 

 other is fitted a narrow glass tube connected with the apparatus 

 designed to produce the requisite difference in pressure. 



*' It is impossible to employ any of the air-pumps at present in 

 use for this purpose, since the filtrate not unfrequently contains 

 chlorine, sulphurous acid, hydric sulphide, and other substances 

 which would act injuriously upon the metallic portions of these 

 instruments. A water air-pump is, therefore, emplo3'ed, con- 

 structed on the principle of Sprengel's mercury-pump, by con- 

 necting the flask which receives the filtrate with a vertical pipe 

 through which a stream of water continually flows. This stream 

 of water sucks down with it the air from the flask in a continuous 

 series of bubbles. With a fall of from 30 to 40 feet, the rarefac- 

 tion can be carried to within 8 or 12 millimetres of a perfect 

 vacuum ; but even with a fall of 10 feet only, a great advantage 

 is gained, 



" Bunsen gives a number of examples to show the eflSciency 

 of this new method, from which it appears that the time neces- 

 sary to filter and dry a quantity of sesquioxide of chromium, 

 hitherto requiring about 7 hours, is reduced to 13 minutes ; and 

 the total length of time needed to filter the sesquioxide of chro- 

 mium, wash and dry the precipitate, and evaporate the filtrate, is 

 reduced from 14 or 15 hours to about 32 minutes. 



"Other advantages gained by this method are that the pre- 

 cipitate is obtained so dry as that it can be immediately ignited, 

 or, if necessary, it can be reaxlily removed from the filter without 

 loss, and without being mixed with jxirticles of the paper. More- 

 over, if the filtrate is to be used for further investigation, there is 

 a much smaller bulk to be evaporated than if the precipitate had 

 been washed in the old way." 



