Report on the Bacteriology of Water. 407 



the colonies are crowded together on the plate, very probably, also, 

 the nature of the other colonies on the plate, and certainly the degree 

 of vitality possessed by the typhoid bacilli themselves. Thus, if the 

 numerical estimate of the typhoid colonies on a plate is made by 

 counting as such the characteristic surface expansion colonies only, 

 the result must be entirely fallacious, as nothing is, in my experi- 

 ence, commoner than to find only a vanishing proportion of the total 

 number of typhoid colonies, even on a pure plate, giving rise to these 

 surface-expansions at all. 



In. looking for typhoid bacilli in such artificially-infected unsterile 

 waters it is, in fact, necessary to employ special methods for their detec- 

 tion similar to those which, as already pointed out, had to be devised 

 for the examination of natural waters for the typhoid bacillus, and it is 

 only when such special methods have been employed with a negative 

 result that the conclusion can be legitimately drawn that the typhoid 

 bacillus is not present in the living state in the particular volume of 

 water operated on. 



In the present investigation, the uniform practice has been made 

 of examining all unsterile waters by means of Parietti's method of 

 phenol broth-culture (see description below) in order to ascertain 

 the presence or absence of the typhoid bacillus or of the B. coli com- 

 munis. 



Method of Detecting the Typhoid Bacillus and Bacillus Coli Communis 

 in Unsterile Waters. 



It will not be necessary to describe the various methods which 

 have been devised for discovering the presence of typhoid bacilli in 

 water, but it will be sufficient to point out that these are nearly all 

 based upon the fact that the typhoid bacillus is, in comparison with 

 most bacteria, but little affected by small doses of either phenol or 

 dilute acids, so that, by adding suitable quantities either of phenol 

 alone or of phenol in conjunction with acid to the culture-media, the 

 growth and multiplication of the typhoid bacillus is not materially 

 interfered with, whilst the proliferation of most, if not of all, the 

 water-bacteria is suppressed. 



Of these various methods, the one which I selected for the purposes 

 of this investigation was that devised by Parietti (" Metodo di ricerca 

 del Bacillo del tifo nelle aque potabili," ' Kivista d' Igiene e Sanita 

 pubblica,' 1890). This method, which consists in adding phenol 

 along with hydrochloric acid in certain proportions to neutral broth 

 is carried out as follows : 



A series of test-tubes containing 10 c.c. of neutral broth, each 

 receive from 3 to 9 drops of a solution having the following composi- 

 tion : 



