CH. V-] 



WOOD-OST METHOD. 



133 



asbestos (see note 2) is poured on to the plate (or into the Gooch 

 crucible) and allowed to settle down without suction. After a short 

 time the tube E (fig. 9) is connected to the filtering flask, the tap C 

 being turned so that it connects to B (thus practically preventing 

 suction through E), and the water pressure turned on. The tap C 

 is then turned through an angle so as to allow suction through E, 

 but before the water has been completely drained off, the tap is 

 rapidly opened again, it being important not to suck too hard. The 

 amount of asbestos required is such as to form a mat about 2 mm. 

 in depth. A small porous plate may be placed on the top of the mat 

 to prevent the latter from being disturbed too much by pouring on 

 water or the copper solution. The mat is then washed two or three 

 times with distilled water, gentle suction being applied after each 

 addition. The final suction should be sufficient to make the mat 

 quite firm. The mat should be prepared during the heating process, 

 it only requiring a few minutes, in spite of this long description. 

 The cold copper solution is poured on to the upper porcelain plate 

 and suction then started. The pressure must be released before all 

 the solution has passed through, it being most important to avoid 

 caking the cuprous oxide by too high a pressure. The flask is 

 washed out with about 10 cc. of cold distilled water (which may, 

 with advantage, have been recently boiled and cooled), and this 

 poured on to the cuprous oxide on the asbestos and filtered through 

 as before. This washing is repeated twice more, care being taken 

 all the time to prevent caking of the cuprous oxide by too high a 

 nitration pressure. 



Solution of the precipitate. Measure 25 cc. of the ferric sulphate 

 solution by means of a measuring cylinder. Pour about 5 cc. of this 

 into the boiling flask and shake this round to dissolve any cuprous 

 oxide that is sticking to the walls of the vessel. Transfer the filter- 

 ing mat and the filtering discs to a small beaker by means of a small 

 glass rod that has a pointed hook. Remove the funnel from the 

 filtering flask and wash it down into the beaker with the remainder 

 of the acid ferric sulphate. Wash out the flask two or three times 

 with small quantities of water,transf erring this to the beaker through 

 the funnel. Wash down the small rod and stir well with a larger 

 glass rod, which should not be guarded with a rubber collar, since 

 permanganate attacks rubber. The cuprous oxide may all dis- 



