492 PEACTICAL PHYSIOLOGY 



disc, bevelled so as to fit the funnel accurately. This disc is covered with a 

 circularly cut piece of filter paper of slightly larger diameter than the disc, so 

 that its margin lies on the funnel. A suction pump (such as that of Bunsen) 

 is attached, by pressure tubing, to the side tube of the flask, and the fluid to 

 be filtered is poured on to the filter. The suction causes the edge of paper to 

 adhere to the glass of the funnel, and the fluid filters perfectly clear. 



3. Weighing Filters. These are employed where it is desired to weigh a 

 precipitate. The simplest form consists of a small hardened filter paper which 

 has been folded and placed on a watch glass in a desiccator for several hours, so 

 as to dry it. To weigh it, a second watch glass is placed over the one holding the 

 filter paper, and the whole is transferred to the scale pan. After weighing, the 

 filter paper is fitted to a small funnel, and the precipitate collected on it removed 

 to a watch glass, dried in a desiccator, and weighed as before. The difference in 

 weight gives the amount of precipitate. 



Another method is to make an asbestos filter, dry it and, after weighing, to 

 filter the fluid through it under suction. When filtration is complete, the preci- 

 pitate is washed, dried and weighed. An asbestos filter is made in the following 

 way. A piece of glass tubing of 1^-2 cm. diameter is drawn out to a tube of 

 about 3 mm. width. The wide portion should be from 6-8 crn. long, and the 

 stem about 4 cm. The neck, where the stem and wide portion meet, is loosely 

 plugged with cotton wool and, above this, is placed loosely packed asbestos threads 

 for about 1-1| cm. The asbestos should have been previously purified by boiling 

 it with strong alkali and acid and then washing with water. The plug of cotton 

 wool prevents any pieces of asbestos being sucked out of the filter. The suction 

 pressure employed with these filters should be gradually applied as, otherwise, 

 some of the precipitate may be sucked through them. 



4. Separating Funnel. This is used for separating two fluids which do not 

 mix. In physiological chemistry it is mainly employed for separating ethereal 

 and watery solutions from one another. Its shape will be seen in Fig. 279. While 

 shaking ether, it is necessary frequently to open the tap so as to prevent the 

 stopper being blown out. Before doing this, the funnel is, of course, inverted. 

 To separate water and ether, the funnel is placed in an upright position so as to 

 allow the water to sink. When a sharp line forms between the two fluids the 

 stopper is removed and the tap turned so as to drain off the water. When nearly 

 all this has been removed, the tap is almost closed so that the outflow may be the 

 more easily controlled. After all the water has been removed, the ethereal 

 fluid should be shaken, whereby, it will often be found, more water separates out. 



Desiccators. There are various forms of these. The two types represented 

 in Figs. 280, 281 are perhaps the most useful, the former being employed for 

 desiccation alone, the latter for desiccation in vacuo. In the conical under-part 

 of the desiccator depicted in Fig. 280 is placed some highly deliquescent substance 

 such as concentrated sulphuric acid or fused calcium chloride. A piece of wire 

 gauze is placed on the floor of the upper chamber, and on this rests the vessel 

 containing the substance to be dried. A glass lid with ground-glass edges is 

 fitted on to the ground-glass edge of the upper chamber, the junction being made 

 air-tight by smearing the applied surfaces with vaseline or resin ointment. The 

 other desiccator (Fig. 281) is provided with an opening whereby it can be 

 connected, by means of tubing, etc., with a suction pump. The deliquescent 

 substance is contained in a vessel placed in the lower chamber. This vessel 



