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



gentle pressure, the proper shape is completed. The platinum 

 funnel, which should not allow of the transmission of light 

 through its extreme point, even now possesses such stability 

 that it may be immediately employed for any purpose. If de- 

 sired, it may be made still stronger by soldering down the over- 

 lapping portion in one spot only to the upper edge of the foil by 

 means of a grain or two of gold and borax; in general, however, 

 this precaution is unnecessary. If the shape has in any degree 

 altered during this latter process, it is simply necessary to drop 

 the platinum funnel into the hollow cone and then to insert the 

 solid cone, when by one or two turns of the latter the proper 

 form may be immediately restored. The platinum funnel is 

 placed in the bottom of the glass funnel, the dry paper filter 

 then introduced in the ordinary manner, moistened, and freed 

 from all adhering air-bubbles by pressure with the finger. A 

 filter so arranged and in perfect contact with the glass, when 

 filled with a liquid will support the pressure of an entire atmo- 

 sphere without the least danger of breaking ; and the interspace 

 between the folds of the platinum-foil is perfectly sufficient to 

 allow of the passage of a continuous stream of water. 



In order to be able to produce the additional pressure of an 

 atmosphere, the filtered liquid is received in a strong glass flask 

 instead of in beakers*. This flask is closed by means of a doubly 

 perforated caoutchouc cork, through one of the holes of which 

 the neck of the glass funnel is passed to a depth of from 5 to 8 

 centimetres (fig. 3) ; through the other is fitted a narrow tube 

 open at both ends, the lower end of which is brought exactly 

 to the level of the lower surface of the cork, to the other is adapted 

 the caoutchouc tube connected with the apparatus (fig. 4) de- 

 stined to produce the requisite difference in pressure : this appa- 

 ratus will be described immediately. The flasks are placed in a 

 metallic or porcelain vessel (fig. 3), in the conical contraction of 

 which several strips of cloth are fastened. This method of sup- 

 porting the flask has the advantage that, in one and the same 

 vessel, flasks varying in size from 0*5 to 2*5 litres stand equally 

 well, and that, by simply laying a cloth over the mouth of the 

 vessel, the consequences of an explosion (which through inexpe- 

 rience or carelessness is possible) are rendered harmless. 



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

 use to create the difference in pressure, since the filtrate not un- 

 frequently contains chlorine, sulphurous acid, hydric sulphide, 

 and other substances which would act injuriously upon the me- 

 tallic portions of these instruments. I therefore employ a water 



* These flasks must be somewhat thicker than those ordinarily used, in 

 cider to prevent the possibility of their giving way under the atmospheric 

 pressure. 



