62+ THE POPULAR SCIENCE MONTHLY. 



the alluvial soil of the Salzbach and the Inn, as Munich stands on the 

 Isar ; but the first-named towns have about fifty per cent more rainfall 

 than Munich. I can only imagine that the necessary degree of dryness 

 for the development of cholera would be attained but very rarely in 

 Salzburg and Innsbruck, just as occurred partially at Lyons in 1854, 

 and in June, 1859, at Bombay, where cholera prevailed during the mon- 

 soons, which, as a rule, drive the cholera away. 



The disposition of cholera in regard to time is also evidenced in the 

 fact that the disease is so different in one and the same place at differ- 

 ent times, or at like times in one and the same place, if different parts 

 of the place have different kinds of soil. For instance, in Munich the 

 houses situated on the clay ridge of the suburb of Ilaidhausen are 

 never affected, but this exemption is certainly not due to the supposed 

 j)revention which clay soil in and of itself exerts against the develop- 

 ment of the germs of cholera, but because the behavior of rain on clay 

 and rocky soil is very different if the rain be equally distributed over 

 the two kinds of soil. When the rocky soil at Munich was ready for 

 cholera, the clay soil was not. 



I shall now leave the arguments for the localists, and pass on to 

 consider the circumstances which are favorable to the views of the 

 contagionists. 



That an epidemic of cholera does not permanently last in one place, 

 but after a longer or shorter time ceases, is explained by the conta- 

 gionists as due to the saturation of the population, whereby each indi- 

 vidual acquires a protective influence against cholera similar to that 

 acquired after vaccination as against small-pox and other like instances. 

 This hypothesis does not explain why an epidemic is sometimes rapid 

 and sometimes slow in its course, why it is sometimes vast in its rav- 

 ages and at other times slight in its effects, while the condition of 

 mankind remains practically the same. With as much reason might 

 the localists assert that the germs of cholera find at different times the 

 local conditions to be favorable or unfavorable with the natural con- 

 sequences of growth or death. Now, in districts where cholera is en- 

 demic, as in the soil of Lower Bengal, it is easy to suppose that at one 

 time the conditions for the multiplication of the germs are present, 

 while at another time the opposite state prevails. The dormant condi- 

 tion of the germs must, for a limited time, frequently exist in districts 

 outside India. This supposition will not explain the occurrence in the 

 low-lying parts of Munich of the severe winter epidemic of 1873-'74, 

 after the summer epidemic in the higher parts of the town had ceased. 

 It follows that we must suppose that the germs which give rise to an 

 epidemic may arrive at a place and there exist for some time (in Mu- 

 nich for three months) without showing any manifestations ; and 

 that, indeed, the germs may die before the necessary local conditions 

 for their growth and multiplication are present. So that one might 

 seek in vain to trace the connection between cases of cholera coming 



