380 



H. M. Vernon 



might possibly have a similar effect. To test this point, a small 

 quantity of the deposit, amoimting in all to only one or two grams, 

 was dredged up from one of the reservoirs, and was stirred from 

 time to time with about 700 cc. of water, for half an hour. After 

 Standing a few minutes, the water was liltered off and analysed, It 

 was found to contain .657 mgm. of free, and 1.490 mgm. of organic 

 ammonia, in place of the respective amounts of .286 mgm. and 

 .157 mgm. originally present. So far from this deposit being a puri- 

 fying agent, it is therefore a most fertile source of contamination. 

 Thus the deposit had been collecting for only two months, previous 

 to which the reservoir had been thoroughly cleaned out. Probably 

 if it had been standing a longer time, its fouling properties would 

 have been miich greater. 



On comparing the effect on larvai growth of water drawn from 

 one of the pipes with that taken directly from the large tank of the 

 Aquarium, the increase in physiological purity proved to be no less 

 marked than the increase in chemical. l'he following were the results 

 obtained. 



In these three experiments the larvai were on an average in- 

 creased 7.8^ in size, but the percentage of ova reaching the larvai 

 stage was only very slightly increased. In each case the water con- 

 tained about 40^ less free ammonia, but the organic ammonia was 

 but little altered. 



The tap water is also bacterially purer than the ordinary 

 tank water. Thus in one case, after 24 hours' incubation, the tap 

 water was found to give 930 colonies, as agaiost the 2(100 of the 

 normal tank water. In another case, after 24 and 48 hours' incubation, 

 the tap water gave respectively 580 and 700 colonies, and the nor- 

 mal tank water 1500 and 3340. The number of bacteria seemed 

 also to depend upon the rate at which the water flowed from the 

 tap; but as the results obtained were rather contradictory, it is 

 deemed unnecessary to mention them in deta,il. 



