112 



EXAMINATION OF AIR AND WATER 



Allow the gelatine to set ; and incubate at 22° C. for as long as 

 possible before complete liquefaction occurs. Count the colonies 

 which appear after forty-eight hours' incubation, take the average 

 at the period of maximum growth, multiply up according to the 

 raction of a c.c. which has been used, and return as so many 

 organisms per cubic centimetre.^ It is necessary that each quantity 

 of water from which the fractional part is added to the gelatine 

 should be taken up separately, and not that i c.c. of water should 



* The number of bacteria per c.c. of course varies within wide limits. As 

 an illustration the official returns may be given of the Metropolitan Water 

 Supply for the first six months of 1903 (as reported by Sir W. Crookes and 

 Professor James Dewar) : — 



For January the Water Examiners' report : — 



" Of the 298 daily samples taken from the filter wells of the Metropolitan 

 Water Companies and examined bacteriologically by us, 13 samples, or 4-3 

 per cent., were sterile ; 45 samples, or 15-1 per cent., contained more than 100 

 microbes per c.c. ; and 21 of these samples contained more than 150 microbes 

 per c.c. The mean number of microbes in the 45 samples containing more 

 than 100 microbes was 198, against a corresponding mean of 177 in 23 samples 

 in December. 



" Owing to the great increase in the rainfall during the last month, the 

 microbes in the unfiltered Thames water have risen from about 6,000 to 13,000, 

 that is, the bacteriological impurity has about doubled, whereas the unfiltered 

 New River water has undergone little or no alteration. The result of this 

 increase has been that the filters of the Thames-derived Companies, which 

 were not working at their best, furnished a larger number of samples from the - 



