The most insidious procesa ia that of the gradual .: fouling of the semiporous enrth lying between the soiiroe of the impurity and the driDking-water well. In auch cases the exudation ia usually quite or ne rly constant ; thore is no opportunity for the air to restore the filtering power of the soil, and it becomes saturated with impurity inch by inch, until, perhaps after a month or perhaps after several yeara, the aaturation roaches the well ; then every drop oozing in f rom this aource carriea with it its atom of filtli. While the aupply of water in the gro.nd is copious, and while there ia more or less circulation through the water veins, the foulness may be too much diluted to do harm ; but in dry seasons, wlien the aupply recedes to a depth of only a few foei at the bottom of the well, the contribution of drain water coptinuing the same, the dose becomes suJficient to produce its poisonous effect. The dangerous charactor of the water of such wells is often manifested by no odor or taste of organic matter ; the chemical changes in this matter seem to have been carried ao far as to yield little more than vivifying nitrates to the water, their organic charactor having entirely d3appeared. Indeed some of the most dangerous well-waters are especially sparkling and refreshing to the taste. But tho chemical processes wliich have effected this chango appear to have had no effect on the germs of disease - if germa they be- which fully retain their injurious character.