1885.] The Removal of Micro- Organisms from Water. 385 



The rate of filtration was exceedingly rapid, viz., 1 inch in less 

 than two minutes, or more than 30 inches per hour. The number of 

 organisms found in the unfiltered and filtered waters respectively 

 were — 



Unfiltered water 11,232 centres per c.c. 



Filtered „ 1,012 



Thus already at the outset the silver sand filter fails to arrest all 

 the organisms in the water, although it very considerably diminishes 

 their number, even when the rate of filtration is exceedingly rapid. 



Powdered Glass. — This material also was employed, with a view to 

 determining the value of non-ferruginous siliceous matter. The rate 

 of filtration in this case was also very great, viz., 1 inch in less than 

 eight minutes. 



The following results were obtained : — 



Unfiltered water .... 11,232 centres per c.c. 

 Filtered „ 792 „ „ 



The results obtained with the powdered glass very closely resemble 

 those obtained from the silver sand ; but as neither were efficient 

 filters, even at the outset of their action, no further experiments were 

 made with them. 



Removal of Micro-organisms by Agitation with Finely Divided Solid 



Matter. 



The above experiments show that micro-organisms may be more or 

 less completely removed by mere contact with solid particles, some 

 of which are incapable of exerting any chemical action upon them ; 

 it therefore appeared of interest to ascertain whether these orga- 

 nisms may be removed by agitating the water with the same sub- 

 stances, and then allowing them to subside. The experiments under- 

 taken with this object were conducted in the following manner : — 



50 c.c. of urine-water, in which the number of organisms was 

 determined, were placed in a small accurately stoppered bottle, pre- 

 viously sterilised by being heated to 150° C, for at least three hours. 

 A given weight of the substance was placed in a small test-tube, 

 plugged with cotton- wool, and then sterilised in the same way. The 

 sterilised substance was then transferred to the water in the stoppered 

 bottle, and the latter violently shaken for a definite length of time, 

 after which it was allowed to stand at rest until complete subsidence 

 had taken place. Some of the clear supernatant liquid was then taken 

 out with a pipette, and examined for organisms by means of gelatine 

 cultivation in the manner already described. The substances were in 

 all cases employed in the same state of division as in the filtration 

 experiments. 



