510 THE POPULAR SCIENCE MONTHLY. 



lar fashion does good drainage act in cleansing our towns, and the 

 necessity of a pure water-supply is thus vindicated. It is in this way 

 that, according to my view, cleanliness acts as a deterrent to cholera. 

 Cholera-germs may come, but they can not fructify under such cir- 

 cumstances. That sites naturally exist which, without human inter- 

 ference, are unfavorable to cholera, has already been shown. 



Where water entirely fails the organic processes soon come to an 

 end ; this is true of the soil of the earth. In rainless deserts the soil 

 is dry except the most superficial layer during the night. In such 

 desert places no organic processes can go on ; this is shown not only 

 in the absence of vegetation, but may be proved by an investigation 

 of the nature of the air of the soil (" Grundlutt ") ; this air under ordi- 

 nary circumstances contains much carbonic acid, which proceeds fi-om 

 the processes of organic life ; but where the soil is free from water 

 the air of the soil much more closely resembles that of the atmosj^here 

 above it. This fact has been experimentally proved by Professor von 

 Zittel by a comparison of the free atmosphere with the air of the soil 

 of the Libyan Desert. These observations are believed to exj^lain how 

 it is that cholera does not appear on a very dry soil. Just as too much 

 water is bad for certain plants, so is it also for some members of the 

 lowest class of the vegetable kingdom. It is likewise conceivable that 

 the organic processes in the soil on which epidemics of cholera depend 

 may be effectually checked by an excess of subsoil-water or by a want 

 of material. Micro-organisms have been divided into two classes : 

 anaerobe and aerobe. If now we have to deal with an organism which 

 requires oxygen for its existence (aerobe), it is not difficult to under- 

 stand how the excess of water might deprive the soil of the necessary 

 proportion of air. The more the pores were filled with water the less 

 air would be contained in the soil. In heavy clay soils the water 

 drives the air completely out, and thorough desiccation would be re- 

 quired to replace all the air. Klebs and Tommasi-Crudeli have already 

 discovered a micro-organism which flourishes only in a moist soil con- 

 taining air — the bacillus malarise. 



We shall now inquire into the time relations of cholera at its per- 

 manent home in Lower Bengal. Dr. John Macpherson has, in his 

 work on " Cholera in its Home," tabulated the number of cases of 

 death from cholera in Calcutta for each month of the year for a period 

 of twenty-six years. I have calculated and arranged in a tabular form 

 from these statistics the average number of deatbs in each month, and 

 contrasted each month with the average rainfall at Calcutta. (See 

 Table I.) It will be seen how unequally distributed is the great fall 

 of rain, which is two or three times greater than in many districts of 

 Germany. Calcutta has a rainy season, which begins at the end of 

 May and ends at the beginning of October. The cholera decreases 

 from the beginning and increases again toward the end of the rainy 

 season. It reaches its maximum during the driest and hot months 



