296 MICRO-ORGANISMS IN WATER 



typhoid colonies on a plate giving rise to these surface- 

 expansions at all, so that any estimate of the number 

 of typhoid colonies from the number of such surface- 

 expansion colonies would be utterly fallacious. In our 

 own experiments, which are still in progress, on the 

 vitality of typhoid bacilli in unsterilised waters, we 

 have, therefore, invariably, in examining the waters for 

 typhoid bacilli, submitted them to a preliminary pro- 

 cess of sifting by Parietti's method (see p. 272) of 

 phenol-broth culture, whereby the recognition of the 

 typhoid bacillus, even when surrounded by a motley 

 crowd of water-bacteria, is rendered possible. Of 

 course this method of procedure banishes the possi- 

 bility of forming any estimate of the actual number of 

 typhoid bacilli in the water under investigation, although 

 some idea of the numbers and degree of vitality of the 

 bacilli in the water can be obtained from the length 

 of time which elapses between the inoculation of the 

 phenol-broth and the appearance of turbidity in it. 



Vitality of the Cholera Bacillus in Various 

 Waters 



In the Appendix, p. 399, will be found a more de^ 

 tailed account of the principal microscopical and mor- 

 phological characteristics of Koch's cholera bacillus ; 

 the following fig\ 20 represents the bacilli as seen 

 under the microscope. 



Fig, 20. — Cholera Bacilli from a Pure Culture. 

 (After Jakscb.) 



