Page:Popular Science Monthly Volume 26.djvu/642

624 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 Innsbrück, just as occurred partially at Lyons in 1854, and in June, 1859, at Bombay, where cholera prevailed during the monsoons, 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 different 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 Haidhausen are never affected, but this exemption is certainly not due to the supposed prevention which clay soil in and of itself exerts against the development 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 contagionists as due to the saturation of the population, whereby each individual 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 ravages 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 consequences of growth or death. Now, in districts where cholera is endemic, 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 condition 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 Munich 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