BACTERIOLOGICAL INVESTIGATIONS 115 
Temperature of Incubation 
For gelatine ... ... ... ... ... ... 20" C. 
For carbol agar ... ... ... ... ... 42*^ C. 
For milk ... ... ... ... ... ... 37° C. (aiicTerobically'l 
Time of Incubation 
For gelatine ... ... ... ... ... ... 72 hours 
For agar 24-48 „ 
For milk 24-48 „ 
Dilutions 
In the case of all drinking waters, and in other cases where the number of organisms will 
probably not exceed 300 in i c.c, one cubic centimetre of the water is mixed with the gelatine, 
agar, and milk. 
In those cases where the number or organisms is probably great, dilutions of i in 10, i in 
50, I in 100, &c., are used. 
Use of the Various Media 
The gelatine is used for quantitative analysis only, serving to indicate how many bacteria 
are present in i c.c. of water. The counting is made at the end of 72 hours. The proportion of 
liquefiers to non-liquefiers is not taken into consideration. A collection has been formed of all the 
different organisms isolated from the water samples, and these are kept alive by constant 
re-inoculation. 
The carbol agar^ to which i c.c. of water is added, is for determining the presence or 
absence of the B. coli in that quantity of fluid. The proportion of carbolic acid in the agar is 
I in 1,000 when the i c.c. of water to be tested is added. 
The milk^ to which i c.c. of the water is added, is used for ascertaining the presence or 
absence of the B, enteriditis sporogenes. One cubic centimetre of the water is added to sterilized 
litmus milk and heated at 80° C. for 15 minutes, then incubated at 37° C. an;erobically. 
The bacteriological investigations which have been made since the year 1898 may be 
described as experimental and routine. 
I. The Experimental Investigations 
These consist of the quantitative and qualitative analysis of — 
I. Unfiltered TVaters. 
[a) Samples of the surface moorland of the watersheds. 
(/>) Streams and rivulets running into reservoirs. 
[c] Land drains running into reservoirs. 
(d) Reservoirs, 
{e) Conduits leading the water to the filter-beds. 
The above examinations are conducted on the Riviugton and l^'yrnwy watersheds. 
