558 BACTERIA IN WATER. 



by colored lines, and the number of colonies in several squares is 

 counted in order to obtain an average and estimate the entire 

 number. 



Water which contains numerous liquefying bacteria had better 

 be examined by the use of nutrient agar instead of gelatin; and in 

 very warm weather it will be necessary to use an agar medium, as 

 ten-per-cent gelatin is likely to melt if the temperature goes above 

 22 C. A difficulty in the use of agar for plates consists in the lia- 

 bility of the film to slip from the glass. This may be remedied to 

 some extent by adding a few drops of a concentrated solution of gum 

 acacia to the liquefied agar medium. Petri's dishes are well adapted 

 for the use of the agar medium, as the objection referred to does not 

 apply to them. The gelatin-agar medium, containing 5 per cent 

 of gelatin and 0. 75 per cent of agar, may also be used with advan- 

 tage in the bacteriological analysis of water. Much stress was at 

 one time laid upon the enumeration cf liquefying colonies, upon 

 the supposition that the liquefying bacteria were especially harmful 

 as compared with the non-liquefying, and that a water containing 

 many liquefying colonies was to be looked upon with suspicion. We 

 now know, however, that there are many common and harmless 

 saprophytes which cause the liquefaction of gelatin, and that some 

 of the most dangerous pathogenic bacteria do not liquefy gelatin. 

 This distinction has therefore no special value, and the question for 

 bacteriologists to-day is not how large is the comparative number of 

 liquefying colonies, but what species are represented by the colonies 

 present, liquefying and non-liquefying, and what are the special 

 pathogenic properties of each. The answer to these questions, in 

 the case of any particular water supply, calls for special knowledge 

 and great patience and care in the isolation in pure cultures, and 

 careful study of the various species present. 



It is now generally recognized that a mere enumeration of the 

 number of colonies which develop from a water under investigation 

 is not a sufficient indication upon which to found an opinion as to its 

 potability. An excessive number of bacteria is an indication that 

 the water contains a large amount of the organic material which 

 serves as pabulum for these microorganisms. But the chemists are 

 able to determine the amount of organic matter present in water 

 with greater precision ; and, as we have seen, the number of bacteria 

 may increase many-fold in water which is kept standing in the labo- 

 ratory for two or three days in a well-corked bottle. As a matter of 

 fact, the enumeration of bacteria in water, although it has given us 

 results of scientific interest, has not materially added to the methods 

 previously applied for estimating the sanitary value of water ob- 

 tained from various sources for drinking purposes. But the bacte- 



