CHOLERA. 175 



coldest seasons of the year ; nay, in some cases the appear- 

 ance of the cold damp season is the signal for the outbreak 

 of a cholera epidemic, because not until the return of the 

 cold weather, after the hot dry summer and autumn months, is 

 there that dampness of soil and atmosphere which is essential 

 for the existence of the organism, in addition to which the soil 

 remains at a comparatively high temperature for some time 

 after that of the atmosphere has fallen. Again, the external 

 cold, though it may paralyse the organism for a time, does 

 not destroy its vitality, but, keeping the organism in a 

 passive condition, actually enables it to retain this vitality 

 for a considerable period, so that, when it finds its way into 

 dwellings, in which the atmosphere, owing to bad ventilation, 

 is not only raised to a high temperature, but also contains a 

 considerable quantity of moisture and organic matter, it is at 

 once introduced to conditions that are essentially favourable 

 to its saprophytic existence ; it again begins to thrive 

 luxuriantly, and becomes a possible source of infection. 



The fact must not be ignored, also, that seasonal temperature plays not 

 only a most important part in the determination of the amount of moisture 

 in the air and in the soil, but also in the production of currents of air by 

 which the organisms may be carried from point to point (though these 

 currents, when the air is dry, or in hot, clear weather, play but a small part 

 in the dissemination of the disease), and by its effects on certain kinds of 

 vegetation, which indirectly have been proved to play an important part in 

 the distribution of the cholera organism. 



On the effect of heat on the amount of moisture in the 

 atmosphere, and consequently on the depth of the ground 

 water, it is scarcely necessary to speak at any great length, 

 but there can be little doubt, from Pettenkofer's observations, 

 that such factors do play a most important part in condition- 

 ing the growth and multiplication of the cholera organism. 

 The rapid removal of stagnant and upper ground water (if 

 complete) is inimical to the saprophytic growth of the 

 cholera organism, as even organic matter in a state of dust is 

 absolutely useless as a nutrient material for the cholera 

 bacillus. Where, on the other hand, there is a considerable 

 quantity of moisture in the atmosphere, even though the 

 temperature be high, stagnant water is often found. 



Koch says : " On the surface or in the ground, in marshes, in docks, which 

 have no outlet, in places where the ground is formed like a trough, in sluggish 

 rivers and the like . . . there a constant nutrient solution can be formed and 



