694 BACTERIA IN AIR, SOIL, WATER, AND MILK 



zontal surface and transfer to the 37 C. incubator as soon as it is hard. 

 Incubate the culture for 24 hours at a temperature of 37 C. in a dark, well- 

 ventilated incubator where the atmosphere is practically saturated with mois- 

 ture. 1 After the period of incubation place the Petri dish on a glass plate 

 suitably ruled and count the colonies with the aid of a lens which magnifies 

 at least five diameters. So far as practicable the number of colonies upon the 

 plate shall not be allowed 'to exceed 200. The whole number of colonies upon 

 the plate shall be counted, the practice of counting a fractional part being 

 resorted to only in case of necessity. 



"It will be found advantageous to use Petri dishes with porous earthenware 

 covers in order to avoid the spreading of colonies by the water of condensation. 2 



" Expression of Results. In order to avoid fictitious accuracy and yet to 

 express the numerical results by a method consistent with the precision of the 

 work the rules given below shall be followed: 



"Numbers of Bacteria per c.c. 



From 1 to 50 Recorded as found 



to the nearest 5 



it it it it I/) 



" " " " 25 



" " " " 50 



" " " " 100 



" " " " 500 



" " " " 1,000 



" " " 10,000 



" " " 50,000 



" " " ". 100,000" 



Qualitative Bacterial Analyses of Water. Of far greater importance 

 than quantitative analysis is the isolation of bacteria either distinctly 

 pathogenic, such as the cholera spirillum and the typhoid bacillus, 

 or of other species probably emanating from contaminating sources, 

 such as a B. coli. Unfortunately there are no reliable methods by which 

 typhoid and cholera germs can be isolated from water with any degree 

 of regularity or certainty. Although frequently the isolation of such 

 organisms is possible, a negative result in these cases is by no means 

 eliminative of their presence. 



The isolation of typhoid bacilli from water is very difficult, chiefly 

 because of the great dilution which contaminations undergo upon enter- 

 ing any large body of water. The difficulty of isolating typhoid bacilli, 

 even from the stools of infected patients, makes it clear that such diffi- 



1 Whipple, Tech. Quar., 1899, 12, p. 276. 



2 Hill, Jour. Med. Res., 1904, N. S., 8, p. 93. 



