THE WATEK-SUl'PLY OF TOWNS IN THE TUOPICS 297 



B. cluac;>: and ijniHlhal may bo taken as examples, while, between the two groups, come 

 intermediate species such as H. lacfis HH-oijcnei^. These then are the three classes* into 

 which Cleuiesha divides organisms which are of importance in the bacteriological 

 examination of water and many of which, let it be reineml)ei-ed, are included in 

 Houston's elastic term, true B. coli. 



Let us see how he applies his classification to practical purposes, i.e. to establishing 

 bacteriological standards for drinking-waters in India. 



"All we claim to have established," he says, "is, that in any naturally occurring 

 mixture of micro-organisms, a certain proportion (how great a proportion we have at present 

 not much idea), of one or more species will be found to survive the others when the mixture 

 is exposed to prolonged sunlight. But it is with this resistant residuum that the sanitarian 

 in India is most frequently called upon to deal. Hence, the great importance and utility of 

 this classification." He believes, that in a river in the Tropics, the susceptible individuals 

 among organisms will die off with great rapidity and that the bacilli, which the analyst 

 finds, can be compared with those whose powers of resistance have been studied in the 

 experiments quoted. It is, therefore, necessary that the analyst, besides making himself 

 fully acquainted with local conditions, should always ask himself, " What chances has this 

 sample had of being exposed to the action of sunlight?" As regards the waters themselves, 

 Clemesha considers the three groups as follows : — 



1. Tliofc CAiiitainintj (UasK I. — The samples which contain coli commiini.i and the 

 members of the susceptible group, present very little difficulty in arriving at a conclusion, s.^'^d'^'d^f"^'' 

 as regards the undesirable nature of the pollution. In such samples, derived from river Indian waters 

 or lake, there can be no doubt we are dealing with recent, and therefore objectionable, 

 contamination. At the same time, if fsecal contamination has been recent, susceptible 

 organisms (those in group 1.) u-ill be mixed with those of groups 2 and 3, roughly in the 

 proportion in u-hirJi theij occur in fmces. Hence we have the double reason for suspecting 



the water; namely, (1) tlie presence of susceptible organisms, which shows that the 

 contamination must have been recent and (2) the confirmatory evidence of getting the 

 unaltered proportions of fjEcal organisms. (J In actual practice, coli communis is so seldom 

 met with, that the latter condition of affairs is often all we have to guide us. In rivers, 

 organisms in Class I. are more commonly met with, because pollution is constantly being 

 added all along the banks, and, when found, certainlj- indicate pollution of a most serious 

 and dangerous nature. 



2. Those containing Glass JTI. — It is probably not realised how extremely often, in 

 a tropical country, organisms, belonging to Class III. alone, are found in water samples. 

 Personally, we doubt whether such a condition of affairs is ever recorded in Europe. But 

 a glance at the analyses of the several supplies, during the quarter of July, August and 

 September, is absolutely convincing on this point. It is quite common to obtain samples 

 of water which contain nothing else but Bacillus yrnnthal or Bacillns cloacae, or a mixture 

 of the two. ^Yhen this occurs, one may be practically sure that we are dealing with a 

 water from a tank or river that is getting low, and also that these organisms represent 

 the residue of a pollution, months old. 



3. Those containing- mixtures of two or .more classes. — The chief difficulty that will be 

 met with by the analyst using this method, will be to arrive at an opinion from samples 

 showing a mixture of organisms in Class II., or a mixture of Class II. and Class III. It is 

 in dealing with such samples that it is absolutely necessary to know the particulars 

 concerning rainfall, etc., that have been enumerated in the early part of this chapter. 



* Vide Captain Archibald's paper, p. 319 



