■^42 Cholera in J^elation to Certain Physical Phenomena. [part i. 



Although the water-level, soil-ventilation and humidity appear to be the most 

 influential conditions in relation to cholera in Calcutta, there are certain phenomena 

 of prevalence which they do not appear to be capable of explaining. During December 

 and January the prevalence ought, in so far as influenced by them, to be higher 

 in place of lower than in November. The question of the causation of the diminution 

 in prevalence at this time has been already alluded to in those sections of this report 

 treating of temperature and humidity, and it was then pointed out that the influence 

 of temperature probably manifested itself in the phenomenon. The phenomena of 

 prevalence presented by April and May are, perhaps, the most difficult of explanation 

 of any throughout the entire course of the year. Here there is diminished prevalence 

 with slight increase in the depression of the water-level, and with only very slight 

 increase of atmospheric humidity or of obstruction to soil-ventilation. It certainly 

 seems improbable that such small alterations in these conditions should be influential 

 in producing such considerable eff'ects. It must at the same time be borne in mind 

 that, in so far as soil-ventilation is concerned, our data are very imperfect. They 

 were obtained from observations conducted during an exceptional year, when the rainfalls 

 of April and May did not stand in their normal relation to one another, that of 

 the former month amounting to 1*20, and that of the latter to only 1-16 inches. 

 There are good grounds for regarding rainfall as calculated to obstruct soil-ventilation, 

 ' indeed we have experimental proof that it actually does so in Calcutta, so that the 

 average difference between the soil-ventilation in April and May is probably greater 

 than our data would lead us to believe. 



The conditions which we are led to regard as most influential may be supposed 

 to act in two ways in favouring prevalence of the disease. Assuming that cholera is 

 produced by a specific material, or a combination of materials, the conditions of soil- 

 ventilation, of water-level and humidity may influence either the diffusion and 

 preservation of the material, or they may influence its production. The material 

 may be produced either by chemical processes taking place independently of organic 

 influences, or by processes dependent on such influences for their existence. If the 

 "latter be the case, conditions of soil-ventilation, of water-level and humidity, however 

 influential in aiding development and diffusion of the material, may not alone be 

 capable of determining the precise period of maximum production. This may be 

 partially dependent on the intrinsic properties of the bodies in the course of whose 

 organic changes the material is developed, and if so, a precise correspondence between 

 the phenomena of prevalence and of the meteorological conditions favourable to it need 

 not necessarily invariably occur. It is at present, however, premature to speculate 

 more on the matter. The whole subject requires further investigation. All that can 

 be done in the meantime is to suggest a possible explanation of certain of the obscure 

 phenomena of cholera prevalence, and to indicate the extremely complex nature of the 

 questions to be determined. 



