3S2 H. M. Vernon 



It is difficalt to say to what this very marked increase in the 

 physiological purity of the water was due. Thns we have previously 

 Seen that filtratioii of the water throngh pure sand which eontained 

 DO vegetable growth, caiised an increase of only 1.8^ in the size 

 of the larvse grown in it. Such filtration must have removed all the 

 minute forms of animai and higher vegetable life present in the water, 

 and also a certaiu proportion of the bacteria, and should, one would 

 think, have acted more efficiently than filtration through a thin layer 

 of asbestos. Thus determinations by means of piate cultures seemed 

 to show that the asbestos filter removed practieally no germs from 

 the water. In one case the water before filtration was found to 

 contain respectively 800 and 850 colonie» after 24 and 48 hours in- 

 cubation, whilst after filtration the numbers were 1000 and 1820. 

 In another case the numbers were respectively 380 and 6S70 for 

 24 and 48 hours incubation before filtration, and 700 and 12700 

 after it. However in the two instances given a few pages back, in 

 which, after filtration through asbestos, the water was kept for 24 to 

 54 days in darkness, the number of bacteria present is considerably 

 smaller than in the unfiltered water kept under similar conditions. 

 It is probable that the filtration removes a large number of bacteria 

 from the water, but in that particles of organic matter, swarming 

 with bacteria, get caught in the interstices of the asbestos fibres and 

 brokeu up by the rapid stream of water flowing past them, the 

 apparent number in the filtered water is just as great, or is increased. 

 Thus in a gelatiu piate culture, a particle of organic matter con- 

 laining hundreds of germs would give rise to only one colony, just 

 like a single isolated g-erm. 



d. The Effect of Heating the Water. 



Inasmuch as exposure of the water to sunlight, and filtration 

 through asbestos, whereby in all probability many of the bacteria are 

 removed, were found to have such a favourable influence on the 

 growth of the larvai, it was determined to try the effect of killing 

 off the germs by previously heating the water. By this procedure 

 the condition of the water is altered in other respects besides, as all 

 the animai and vegetable life in the water are killed off, in addition 

 to most of the bacteria. Also a large proportion of the gases dissolved 

 in the water is driven off. However, as has been shown in the 



