32 WATER 



It will be noticed that from the first two methods deductions 

 only can be drawn as to purity, while with the last method the 

 actual disease-producing organisms are isolated. 



Quantitative Exam in at ion. 



As almost universally practised at the present day, this 

 consists in adding varying quantities of the water to tubes of 

 liquefied gelatine and agar, each of which is then thoroughly 

 mixed and poured out with all precautions into a Petri dish, 

 and solidified as rapidly as possible. These plates (Petri 

 dishes) are then incubated, the agar at 37 C., and the gela- 

 tine at 20 C. to 22 C. Each organism present, capable of 

 multiplication under the conditions existing, develops into a 

 mass of bacteria or colony, visible to the naked eye, and as 

 such readily counted. The total number of colonies gives 

 the maximum number of organisms capable of development 

 in the medium used, within the time given and at the particular 

 temperature of incubation. 



In actual practice many details have to be attended to. 

 For instance, the reaction of the gelatine and agar used has a 

 marked influence on the number of organisms which develop. 

 Both the English and American Committees on Standard 

 Methods of Water Analysis recommend a + I per cent, reac- 

 tion, and accurately standardized media of this reaction should 

 alone be used for routine work (see Appendix). 



The composition of the agar and gelatine nutrient media 

 also influences the number of colonies, and care should be 

 taken to consistently manufacture in the same way and with 

 the same quality and amount of ingredients. 



The amount of water to add to the media tubes must 

 obviously vary with the suspected degree of contamination of 

 the water. 



For ordinary waters O'2 and 0*5 c.c. are convenient amounts 

 to add to the gelatine tubes, and O'2 and roc.c. to the agar. 

 For contaminated waters a considerably greater dilution may 

 have to be practised. The water is conveniently measured 

 by i c.c. pipettes graduated in one-tenths of a c.c., previously 



