34 SEWAGE AND ITS PURIFICATION 



has been adopted by Dr. McGowan in his work for the Royal 

 Sewage Commission.^ 



A stock of commercial elastic asbestos is shredded, purified 

 by digestion with warm concentrated HCl and washing till free 

 from acid, and stored under distilled water. An even mat of 

 the asbestos 2 to 3 millimetres thick is made on the bottom of 

 a Gooch crucible about if inches high and 1^ inches diameter 

 a.t the top, which is then fitted by means of a ring of wide india- 

 rubber tubing into the neck of a filter-flask connected with a 

 pump. Distilled water is then poured on carefully and drawn 

 through two or three times, and the crucible filter is dried at 

 110° C, cooled in a desiccator, and weighed. Fifty to 500 c.c. 

 of the sample, according to its character, are now filtered, at first 

 without using the pump, refiltering until the liquid passes clear, 

 and reserving the sediment as much as possible to the last. 

 Finally, wash three or four times, dry, cool, and weigh as before. 

 Careful ignition and weighing gives the mineral suspended 

 matters (the odours evolved should be noticed). If necessary, 

 the ash can be chemically examined, as the asbestos is almost 

 unaffected by HCl. 



5. Centrifugal method for approximate comparison of muddy 

 samples. A measured quantity is allowed to settle, and the 

 lower fraction transferred to a tube drawn out into a narrower 

 portion graduated in tenths of a c.c, centrifugalized for a 

 certain time, and readings taken of the volume of moist deposit 

 per 100,000. 



In a full analysis both the dissolved and suspended solids are 

 divided into (i) " organic and other volatile matters," and (2) 

 " inorganic or mineral matters." 



Chlorine is present in the form of chlorides, chiefly of sodium, 

 with less quantities of potassium and ammonium, but is always 

 recorded in terms of chlorine. It is estimated volumetrically 

 with a standard solution of nitrate of silver, adding a drop of 

 neutral potassium chromate, when the appearance of a slight 

 persistent red colour, due to chromate of silver, indicates the 

 complete precipitation of the chloride. It is in all cases 

 advised to evaporate the measured volume of the sewage — 

 10 to 25 c.c. — to dryness on the water-bath before titration ; 

 the end reaction is then sharp. For the standard silver solution, 



^ Fourth Report, 1904, vol. iv., part v., p. 45. Also see an investigation by 

 Kimberley and Hommon (Journal of Infectious Diseases, Chicago, February, 1906), 

 who use the Gooch as the standard method for sewages at the Testing Station, 

 Columbus, Ohio. 



