D. B. WARD, M. D. 195 



the plate is covered up and kept at the ordinary room 

 temperature. In a few days little dots or colonies will 

 appear all over the plate, and these are the progeny each 

 one of a single germ. From the different colonies gela- 

 tine tubes are inoculated, and then we have in these 

 tubes pure cultures of the different species contained in 

 the original mixed culture. This process requires great 

 care and scrupulous cleanliness, and everything must be 

 carefully sterilized to ensure success. Some bacteria will 

 grow only at the temperature of the human body, and 

 of course gelatine will not answer for cultures of them, as 

 it melts at the body temperature. So we can use a jelly 

 made of agar, a Japanese seaweed, which melts at a much 

 higher temperature. For most of the parasitic forms the 

 agar medium is necessary, and the cultures are kept in a 

 brood oven which is constantly — night and day — heated 

 to 98° or 100*^ Fahrenheit. In this way only can the bac- 

 teria of tuberculosis, diphtheria, pneumonia and many 

 others be grown outside the body. The spirillum of 

 Asiatic cholera, however, and the bacillus of typhoid 

 fever will grow readily at the room temperature. A 

 large number of bacteria — mostly saprophytes — wiil not 

 grow at a temperature so high as 100°. There has been 

 in this city during the last two or three years a good deal 

 of discussion with regard to our city water. It may be 

 safely asserted that all drinking water contains bacteria. 

 Now the danger of drinking any particular sample of 

 water does not depend upon the number of bacteria 

 which it may hold but on the kind. Two hundred or- 

 ganisms to the cubic centimetre may be perfectly harm- 

 less if they are simply saprophytes, while one-tenth of 

 that number may be very dangerous if they chance to be 

 the bacilli of typhoid fever, or the spirillum of Asiatic 

 cholera. The problem for us to solve, then, is to find not 

 the number but the kind. Our river is unquestionably 

 the dumping ground for the sewage and refuse of the 



133 



