534 



CHAPTER XXVI 



and above this a pad of asbestos ; the plug of glass wool should be about 

 3 /8 in. deep and the asbestos about i /i6 ; the asbestos pad is most effectively 

 formed by filling the tube with a suspension of the asbestos, and allowing 

 it to settle gradually. It is then drained by the pump, dried, weighed 

 and is ready for use. After the Soxhlet tube has been prepared, it is fitted 

 into the stopper of the filter flask, and filled about three parts full with 

 water ; a small funnel is then fitted on to the tube, the stem of which does 

 not quite reach to the level of the water in the tube. The funnel is then 

 filled with water and the pump started ; as the water passes through the 

 filter, the liquid undergoing filtration is poured into the funnel, care being 

 taken to keep the funnel full. When all the copper oxide has been brought 

 into the funnel, the level of liquid is maintained by hot water until all the 

 precipitate has passed into the Soxhlet tube and is continued until the 

 washing is complete. 



Fig. 344 



Fig. 345 



A Gooch crucible^*' consists of a tall crucible of conventional pattern, 

 the bottom of which is a perforated disc. It is prepared for use as described 

 for the Soxhlet tube, save that the pad of glass wool is unnecessary. 



The filtration apparatus used in the laboratories of the Hawaiian Sugar 

 Planters' Association is shown in Fig. 344. The filter flask is of the form 

 due to Diamond^^. The tube a communicates with the vacuum pump ; 

 connection with the atmosphere may be made by the cock on the tube h. 

 The Gooch crucible c is held in the carbon tube d, a tight joint being made by 

 a piece of inner tubing of a bicycle tyre. The filtrate may be discharged 

 through e. The advantages of this apparatus for all vacuum filtrations 

 are obvious. 



The alundum crucible is made of a porous preparation of ignited alumina. 

 It is prepared for use by boiling in nitric acid. It may be mounted as in 

 Fig. 344 ; but in order to facilitate washing Spencer^- has designed the 

 holder indicated in Fig. 345. 



The spongy platinum filtering surface due to Munro'^ is prepared by 

 igniting ammonium platimun chloride placed on the bottom of a platinum 



