OF ARTS AND SCIENCES. 



345 



cases, asbestus felt is apt to curl at the edges, and without such pre- 

 caution some of the precipitate might drop through the perforations 

 and be lost. For drying at low temperatures, however, and even for 

 ordinary ignitions, a second crucible is unnecessary ; but, during the 

 ignition of an easily reducible :<ul)stance, care must be taken to prevent 

 the contact of unburnt gas with the perforated bottom. 



To perform tlie filtration, the crucible is replaced in the funnel, the 

 pressure applied, and the process conducted precisely as in an ordinary 

 filtration by the liunsen pump. It is necessary to observe that the 

 vacuum-pump is to be started before pouring the liquid upon the filter. 

 The final di-yiiig or ignition, as the case may be, of precipitate and fil- 

 ter is made without difficulty, or need of extra precaution. 



When turbid liquids are to be filtered, or gelatinous precipitates to 

 be separated, instead of the perforated crucible, I 

 prefer to use a platinum cone (Figs. 4 and 5), the 

 upper part of foil (to make a tight joint with the 

 rubber fitting of the funnel), the lower of gauze. 

 The method of covering the gauze with felt is 

 identical with that described above. By leason of 

 the larger filtering surface of this apparatus, the 

 tendency to become clogged is, of course, very 

 much diminished. When subjected to prolonged 

 ignition, the gauze cone is enclosed within a cru- 

 cible or a cone of platinum foil. 



In operations in which platinum is liable to 

 receive injury, a porcelain crucible, with a per- 

 forated bottom, may be used ; but recourse to 

 this is rarely necessary, pai'ticularly when one 

 may use the gauze cone protected, as it is, by 

 asbestus felt; moreover, the perforation of por- 

 celain with numerous fine holes is a matter of 

 considerable difficulty and expense. 



Asbestus felt may be also used in the process 

 of reverse filtering, it being merely necessary to 

 dip the platinum rose into the asbestus mixture, after starting- the vac- 

 uum-pump, in order to make the felt. The rose, with the felt attached, 

 and the vessel in which the precipitate is collected, are to be weio-hed 

 together, both before and after filtration. 



Nothing can be simpler than the whole method of preparation and 

 use of the apparatus which I have described, and its efficiency is 

 extremely great. Clean water may, under the pressure of a Bunsen 



Fig. 4. 



