SPIRILLUM CHOLERA ASIATICS. 457 



2. The nature of the soil is in the first place of im- 2. Influence 



,. ., , ,. ., , of the nature 



portance in so iar as, according to the declivity and O f the soil, 

 porosity of the soil, the rain, the house, and the washing- 

 water, the outflow from water-closets, &c., can be readily 

 carried off, or is retained in stagnant superficial collec- 

 tions, or in the uppermost layers of the soil. Where, 

 as in the suburbs of Calcutta, we have the presence of 

 tanks, and the artificial elevation of the foundations of 

 the houses, the conditions are very favourable for the 

 collection and preservation of infective material during 

 the dry season of the year. A similar accumulation of 

 filth and of sources of infection also occurs not uncom- 

 monly with us in narrow streets and courts. The influ- 

 ence of the soil may also vary according to the season. 

 The chances are evidently favourable for the spread of Dryness of 

 cholera when a so-called ''drying zone" exists in the 

 uppermost parts of the soil, so that all fluids and rain S0ll> 

 which reach the soil remain in the uppermost dry layer 

 (see Part VI.) ; where there is no drying zone the impuri- 

 ties and any infective agents present are as a rule carried 

 to such a depth that they are no longer present at the 

 surface, and thus many chances of transport to man and 

 dwellings are removed ; when however a drying zone is 

 present all infective organisms, which usually reach the 

 soil in considerable numbers with the dejecta, with the 

 contents of the night- stools, with the water from washing 

 clothes, &c., remain for a considerable time in this 

 upper layer. There the conditions arc favourable for 

 the preservation of the comma bacilli as w r ell as other 

 bacteria ; and hence a source of infection is furnished 

 from which the infective material may be carried either 

 directly to man or to articles of food and other things in 

 a great variety of ways, e.g. by man, animals and objects, 

 and also by insects ; and this source of infection is so 

 much the more dangerous the longer it continues to 

 exist. The presence of a drying zone, which is best indi- 

 cated in our neighbourhood by lowering of the level of 

 the ground-water, is therefore under certain conditions 

 an important seasonal predisposing factor for the spread 

 of cholera, while under other circumstances, such as an 



