204 MICKO-OEGANISMS IN WATER 



The treated water was examined again after 18, 24, 

 48, and 72 hours, and again after 4 days ; and bacteria 

 were only found (3-15 in a c.c.) in the water treated 

 with 0'15 g. alum. The water was quite clear after 

 12 hours. The temperature of the room in which the 

 vessels were kept varied from 8-1 5C. 



In order to ascertain in what manner the microbes 

 might be distributed throughout the layers of water 

 thus treated, a high cylindrical vessel (60 cm.) was 

 filled with water, and 0'4 g. alum added to every 

 litre. 



Water taken from the surface after standing 24 

 hours contained about 20 microbes in 1 c.c., whilst 

 in other similar investigations none were found. The 

 water was also found to be quite sterile at a depth of 

 10, 20, 30 and 40 cm., and even after standing 4 days 

 no organisms were discoverable. 



It would appear that alum acts very deleteriously 

 on water-bacteria, for whilst the untreated water con- 

 tained, after standing for four days, 1,500 microbes in 

 1 c.c., and 6,000 microbes in 1 c.c. of the sediment, the 

 sediment of the alum-treated water contained only 

 20-100 bacteria in a c.c. This would account for no 

 subsequent redistribution of the microbes taking place 

 in the upper layers. In consequence of the good results 

 obtained by the use of alum, these authors have devised 

 a piece of apparatus in which precipitation by means of 

 alum, or other substances, is made to take place, and 

 the treated water subsequently drawn off. 



Lankester 1 has also experimented on this method 

 of water purification, using *25 grm. of alum to 1 litre 

 of water ; the untreated water gave, after standing 24 



1 Evidence given before the Royal Commission on the Metropolitan 

 Water Supply, November 1892. 



