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 Berchtesffaden, 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 constitution, 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 varied soils as those composed of granite, sand-chalk, and shell-chalk, we must suppose that the soil contains in its interstices much organic matter and water. Farmers know how useless pure soil is, whereas the luxuriant 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 excellent 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 abundant 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 drainage was in vogue, diseases dependent on conditions of soil (cholera and typhoid fever) have undergone a striking decrease. Just as a field, when excessively manured, 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 themselves, just as a churchyard after a time becomes purified. In a simi-
