CHOLERA. 509 



as Berlin gravel, and that more than a third of its volume consisted 

 of air-containing pores. It is so soft that it can be cut and sawed like 

 wood. As visitors may purchase wood-carvings from Oberammergau 

 and Berchtesgaden, so one can obtain carved-work of Maltese stone. 

 Tiles cut from Maltese stone find a ready sale in Italy, where they 

 serve to decorate the floors of rooms, where, owing to their porous 

 nature, they are not so cold to the feet as stone tiles. Maltese tiles 

 are as good as wood without being so inflammable. Moreover, of the 

 same stone vessels are made which English sailors use to filter their 

 drinking-water. Turbid water when poured into such vessels filters 

 off as a transparent fluid. It will be readily understood that I now no 

 longer concerned myself as to an explanation when I heard that an 

 epidemic of cholera had broken out at a place which apparently had a 

 compact soil. 



Not only does the physical nature, but also the chemical constitu- 

 tion, of the soil have an influence on the occurrence of cholera — to wit, 

 the presence of organic matter and water. The influence of the soil 

 on the development of infectious diseases can only be understood by a 

 study of the organic processes which take place in it. The processes 

 are eventually dependent on the action of the lowest organisms, which 

 require for their growth a certain temperature, so much water, air, and 

 food-stuffs. In order to explain the occurrence of cholera on such va- 

 ried soils as those composed of granite, sand-chalk, and shell-chalk, we 

 must suppose that the soil contains in its interstices much organic mat- 

 ter and water. Farmers know how useless pure soil is, whereas the lux- 

 uriant growth of plants when the ground is manured is well known to 

 all. These observations are applicable to the lowest plants, the bacteria, 

 no less than to grain and vegetables. The germs of putrefaction 

 and fermentation abound in the free atmosphere, but they only grow 

 and multiply where they find suitable food. The hygienic uses of 

 cleanliness here find their explanation and scientific foundation. The 

 refuse from houses, dissolved or suspended in water, forms an excel- 

 lent nutritive material for the lowest organisms which are so harmful 

 to us. Emmerich has shown that the purest water after being used to 

 clean the floor of a room contains in a very short space of time abun- 

 dant germs of disease, so much so that a drop of it injected under the 

 skin of a rabbit or Guinea-pig is followed by a fatal result. With 

 this dangerous slop-water it is the custom to charge the earth in and 

 about our dwellings. Since man began to live in towns where drain- 

 age was in vogue, diseases dej^endent on conditions of soil (cholera 

 and typhoid fever) have undergone a striking decrease. Just as a 

 field, when excessively manui'ed, does not always remain good for 

 vegetation unless remanured, so is it also with the uncleanliness of the 

 soil in the neighborhood of our houses. As soon as we cease to make 

 unclean — to manure — so soon do our towns begin to purify them- 

 selves, just as a churchyard after a time becomes purified. In a simi- 



