234 



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



Estimated average monthly propor- 

 tion of Carbonic Acid in ISoil-air -v 

 [Inverted curve]. I 



Total monthly Cholera Deaths [38 

 years] . 



— Lowest proportion of Co^ 

 —Highest „ „ 



Diagram 9.— Monthly averages of Carbonic Acid in the Soil-air ((5 feet below the sui-face) and 



Cholera-prevalence. 



The line indicating the carbonic acid in the diagram is reversed like those in the 

 diagrams of humidity, rainfall and water-level, and the figures in the table are to be 

 regarded as only representing the relations borne by the amounts of carbonic acid to 

 one another, and not the absolute quantity present. 



Taking the data as they stand, we find that, during November, December and 

 January, the amount of carbonic acid is high. In February a considerable decrease 

 occurs, and the minimum for the year is reached in JMarch and April. During May 

 a slight increase occurs, continued through June and July, and followed by a rapid 

 rise in August to the maximum in September, after which a decrease occurs, reducing 

 the average for October to an equality with that for August on the one hand, and for 

 November, December and January on the other. 



The first question which presents itself here is, — What are we to regard these 

 fluctuations as representing? We believe that they are to be regarded as aflfording 

 an index to the varying degrees of soil-ventilation present at different times of the 

 year ; in other words, to the varying degree in which emanations escape from the soil 

 into the atmosphere at different times of the year. 



The fluctuations in the amount of carbonic acid in the soil-air must be due to one 

 or other of two causes : (1) variation in the amount produced at different times ; (2) 

 variation in the degree of accumulation of what is formed — variation in the amount 

 retained in the soil. The phenomena of fluctuation in Calcutta appear to be mainly 

 determined by the latter agency. The most conspicuous fluctuations during the course 

 of the year are the rapid decrease of carbonic acid during the months of February and 

 IVIarch and the rapid increase during August and September. The coincidence of 

 increased carbonic acid with the occurrence of rainfall has been amply confirmed by 

 the observations of the current year, and the phenomenon is only explicable as due 

 to the action of the rain upon the ventilation of the soil. That rainfall acting on a 

 finely-textured soil like that in Calcutta should produce such an effect, is only what 

 might have been fairly assumed independent of experimental evidence. The surface-soil 

 of Calcutta consists of layers of loam, sand and clay, but the depth ?iod distribution of 



