292 Cholera in Relation to Certain Physical Phenomena. [part i. 



above the water-level or first impermeable stratum in a locality ; and that, therefore, 

 the development must depend on certain conditions of that layer. These conditions 

 are, admittedly, quite undetermined, but must be supposed to be constantly existent, in 

 greater or less degree, within the areas in which the disease is endemic. According to 

 it, fluctuations occur either in conditions affecting the development of the poison in the 

 soil, or in conditions determining its diffusion from the soil. 



So long as the conditions supposed to secure development of the poison are not 

 distinctly defined, it is evident that an endemic locality affords little field for testing 

 the theory in regard to the relation of prevalence of the disease to influences calculated 

 to favour its production. Attention must, therefore, be mainly directed to prevalence 

 regarded as dependent on diffusion ; but even regarding questions of production, 

 there are certain phenomena of prevalence, which admit of comparison with the theory. 

 In any locality, like Calcutta, where the layer of soil above the water-level is always 

 of comparatively little depth, and of tolerably uniform structure throughout, the theory 

 may be supposed to assume, that, other things being equal, the amount of the specific 

 material developed ought to increase with the mass of generating stratum. From this 

 point of view, the maximum and minimum of prevalence ought to coincide with the 

 maximum and minimum depression of the water-level, which is very much what we 

 have previously ascertained to occur in reality. 



In proceeding to consider cholera-prevalence as an expression of the degree of 

 diffusion of the poison, it must be borne in mind that there are two main channels 

 by which materials developed in the soil may reach human beings in any locality. 

 These are the water and the air occupying the interspaces between the solid con- 

 stituents of the soil. If the water of the soil be regarded as the means of diffusion 

 of the poison, the phenomena of prevalence in Calcutta present the same difficulties 

 to this theory as they do to those previously considered. This, however, is not the case if 

 the air be regarded as the channel traversed by the poison in passing from the soil 

 to the subjects of the disease. On this hypothesis^ maximum and minimum of pre- 

 valence ought to coincide with maximum and minimum of soil-ventilation. According 

 to our data, the maximum of soil-ventilation occurs during March and April, coin- 

 cidently with the maximum of prevalence ; and minimum soil-ventilation occurs during 

 the rainy season, the period of minimum prevalence. So far, however, as diffusion 

 alone is regarded as determining prevalence, the phenomena of prevalence in Calcutta 

 do not coincide accurately with the requirements of the theory. The depression of 

 prevalence in December and January, and again in May, remains inexplicable ; and all 

 that can be positively affirmed is, that the difficulties opposed by the phenomena of 

 periodic fluctuation in the prevalence of cholera in Calcutta to the soil-theory, are 

 less than those encountered by any of the other current doctrines regarding the 

 essential cause of the disease. 



It will be observed, that in the previous pages no special notice has been taken 

 of the great and persistent diminution in the prevalence of cholera in Calcutta during 



