PART I.] Current Theories as to the Causation of Cholera. 291 



of extraneous impurities is facilitated, and on influences determining the relative pro- 

 positions of the impurities to the mass of water containing them. The conditions 

 favouring the entrance of foreign matters in general into the sources of water-supply 

 are precisely those described previously as favouring the entrance of specific materials ; 

 and the conditions favouring concentrations of impurity are those under which the 

 general mass of water is reduced to least bulk. Consequently, according to this 

 theory, the maximum prevalence of cholera in Calcutta ought to occur during the 

 hot and dry season, when concentration attains a maximum, and during the 

 commencement of the rains, when there is great inwash of extraneous impurities, 

 without an increase in the mass of the water, calculated to do more than neutralise 

 this addition. 



The phenomena presented by the cholera-prevalence of the hot and dry months, at 

 first sight, appear to afford strong confirmatory evidence to the theory ; for the season 

 of maximum prevalence occurs then, coincidently with the season of lowest water-level. 

 When, however, the phenomena of individual months of the hot and dry season are 

 examined, the results are not so favourable to it. In May the water-level reaches its 

 lowest, the conditions affecting evaporation being, apparently, more than sufficient to 

 neutralise the effect of the slight excess in the rainfall over that of April. In May, 

 then, the sources of water-supply are reduced to their smallest bulk, and the con- 

 centration of the impurities in them reaches a maximum ; and consequently, according 

 to the theory, the cholera-prevalence of the hot and dry season ought to come to a 

 climax then. On the contrary, however, the prevalence in May is much less than in 

 either of the preceding months. 



The phenomena of June do not show any close agreement with the requirements 

 of the theory. At this time, according to the theory, cholera-prevalence ought to 

 continue at a maximum ; for, although the mass of water in the sources of water- 

 supply undergoes considerable increase, the conditions of rainfall are such as to ensure 

 great inwash of materials from the soil, which has not been purified by any great 

 previous rainfall. June is, however, a month of low medium — not of maximum pre- 

 valence. November, also, presents great difficulties to the acceptation of any such 

 explanation of the phenomena of periodic fluctuations in prevalence. In November 

 conditions facilitating inwash of extraneous materials are at a minimum, and the 

 mass of water is greater than in either June or July. The prevalence in November 

 ought, therefore, to be lower than that in June and July; but, on the contrary, it is 

 very much higher than in these months. 



HU. — The " Soil-theory." The theories which have been considered hitherto assume 

 that the nature of the specific cause of cholera, or, at all events, that the conditions 

 determining the production of the specific cause, are already definitely ascertained. 

 The soil-theory, however, does not go so far; and the difficulty of determining the 

 bearing of our data upon it is proportionately enhanced thereby. All that it definitely 

 affirms is, that the specific cause of cholera is developed in the layer of soil lying 



