172 SCIENCE PROGRESS. 



germ ; and, secondly, places it in conditions favourable for 

 its multiplication which takes place with such amazing 

 rapidity that, even in a few hours, or at most in two or 

 three days, each organism will have created around itself 

 a visible colony of innumerable members — a town in fact 

 comparable to London itself for population. 



By operating upon a known volume of water, such as 

 a cubic centimetre for instance, the number of separate 

 organisms or their spores, in a given volume of the water 

 under investigation can thus be determined. The following 

 is the method now adopted in carrying out Koch's process 

 for the bacterial investigation of drinking water : — 



i. Preparation of the nutritive medium. 



2. Sterilisation of the medium. 



3. Collection of the sample of water in a vacuous tube 



afterwards to be hermetically sealed. 



4. Transport of the sample to the bacteriological 



laboratory, packed in ice to prevent multiplica- 

 tion. 



5. Mixture of a known volume of the water sample 



with the nutrient medium. 



6. Casting of the mixture into a solid plate. 



7. Incubation of the solid plate. 



8. Counting of the colonies. 



9. Examination of separate colonies, or rather of the 



individual members under the microscope. 



Sometimes the cultivations are made upon a plate of 

 the substance called agar which resembles isinglass, and 

 bears a temperature of blood heat without melting. 



In order to ascertain the effect of filtration upon the 

 bacterial quality of water, it is absolutely necessary that 

 the sample should be taken immediately after it has passed 

 through the filters ; for, if it be obtained from the delivery 

 mains in town, that is to say, after the water has passed 

 through many miles of pipes, the rapid multiplication of 

 these organisms, except in very cold weather is such, that 

 a water which contains only a single living organism per 

 cubic centimetre, as it issues from the filter, may contain 



