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



in spite of considerable variations in temperature. The amount of carbonic acid present 

 in November, December and January greatly exceeds that present in April, a month 

 in which the soil-temperature is almost equal to that of November, and considerably 

 higher than that in the other two months. But while the temperature of the soil in 

 November and April is almost equal, the amount of moisture in the soil, and consequently 

 of soil-ventilation, is very different in the two months, the former of which immediately 

 succeeds the cessation of the rains, the latter following a long period of almost total 

 rainlessness. It is, moreover, only by explaining the phenomena of fluctuation as due 

 to soil-ventilation that the results of the Indian observations are brought into accord- 

 ance with those attained in Europe. 



The results of the European observations have been to show that the maximum of 

 carbonic acid in the soil-air occurs coincidently with the maximum temperature of the 

 upper strata of the soil, but in Calcutta the maximum of carbonic acid is clearly connected 

 with rainfall and independent of temperature. In Europe the rainfall is distributed 

 throughout the entire year ; in India it is concentrated into a few months ; in Europe 

 the fluctuations in carbonic acid in the soil-air probably correspond with variations in 

 the amount formed, but in many parts of India the variations in amount of formation 

 are almost entirely obscured by the effects of the varying degrees of soil-ventilation, 

 although certain phenomena, such as the increase of carbonic acid during May and 

 June in Calcutta, may be a partial indication of their existence. 



On proceeding to compare the phenomena of cholera-prevalence with those of 

 soil-ventilation as indicated by the carbonic acid of the soil-air, it appears that the 

 maximum of prevalence coincides with the maximum of soil- ventilation and the minimum 

 of prevalence with obstructed soil-ventilation. Taking individual months, however, the 

 correspondence between the course of the two phenomena is not close or uniform, for 

 we find the same degree of soil-ventilation indicated for August, October, November, 

 December and January ; whilst the cholera-prevalence of the same months varies widely, 

 and a similar phenomenon is presented in May, June and July. 



Whether the coincidences between the variations in soil-ventilation and cholera- 

 prevalence indicate any essential connection between the two phenomena or not, there 

 can be no doubt regarding the importance of the observations demonstrating their 

 existence. They show that, in estimating the influence of the rainy season on conditions 

 of health, its action in effecting soil-ventilation cannot be left out of consideration. 

 Until it be conclusively demonstrated that all emanations proceeding from the soil 

 are inert, the presence of any influences obstructing or facilitating their escape deserves 

 careful consideration in any attempts at the explanation of the phenomena of disease 

 as related to season. Hitherto rain has only been regarded as affecting health through 

 the agency of the water supply or its action in washing the surface of the soil, but 

 its relation to soil-ventilation has almost entirely escaped notice. This has, no doubt, 

 arisen from the fact that the subject has been mainly studied in regions with climates 

 like that of Europe, in which the rainfall is uniformly distributed throughout the year, 



