Prevention and Eradication 83 



(4) The B. cercaricc are not entangled in the tlocculent alum 

 precipitate. They are seen swimming freely in the supernatant 

 fluid twelve hours after the addition of the alum. 



(5) The " vital " layer, formed by the deposition of alum on the 

 surface of the sand and the arrest of bacteria and fungi therein, 

 does not arrest the B. cercaria. These vi^ere found to pass easily 

 through the layer formed by the passage for half an hour of 

 aluminized water taken from the settling tanks of the Cairo Water- 

 works. The same result followed in another test made by passing 

 newly forming alum precipitate on a small area of sand for an hour, 

 thus producing an abnormally thick layer. This too offered no 

 obstacle to the leech-like progression of the cercariae, for they were 

 found actively swimming in the filtrate twenty-four hours later. 



(6) Finally, depth of sand presented no insuperable barrier, for 

 very active cercarife were found in the filtrate of our working 

 model within one hour after their addition to the inflow of 

 aluminized water, a depth of thirty inches of sand having been 

 traversed in this interval. The sand was a sample of that 

 ordinarily used by the Cairo Waterworks. Sand of the finest 

 grain used in filtration was similarly tested and proved inefficient. 



Mechanical systems of filtration, such as the Jewell system, 

 depend therefore solely on the delay they interpose between the dis- 

 chargmg mollusc and the consumer for the amount of protection 

 they afford against bilharziosis. At Cairo the additional delay after 

 the intake of water from the main stream of the Nile is about twelve 

 hours, while at Ismailia under a different system the delay is about 

 twenty-four hours. The uniform dispersal of the cercariae in the 

 filtered water has also to be borne in mind. The "time factor" 

 in the life of the B. cercarice apparently affords a satisfactory 

 explanation of the relative immunity of Europeans in those 

 Egyptian towns where there is both a filtered and raw water 

 supply. 



p {A) Sodium bisulphate is used in " tabloid " 



Sterilization. ^°^''^ *° sterilize water for drijiking purposes. 

 Two "tabloids" are dissolved in a quart water- 

 bottle as a rule. Each "tabloid" contains 16 gr. (1 grm.). This 

 gives a dilution of 1 in 567. In a previous paragraph it was shown 

 that a dilution of 1 in 1,000 was quickly lethal to the bilharzia 

 cercaria. These " tabloids " may therefore be used with safety in 

 bilharzia-infected countries. 



{B) In view of its germicidal value, chlorine 1 in 1,000,000 acting 

 for half an hour, is in common use. From the tabulated effects of 

 chemical reagents on the cercaria it will be seen that this dilution 



