an organic nature, to so low a degree that they shall he quickly changed and rendered harmless by the soil. The ground has a certain power to purify itself.
Falck made the discovery not long ago that water containing infectious matter, formed and unformed ferments, organic poisons, etc., ran off pure after having been filtered through very thin beds of sand. At first, the ground only absorbs, as, for example, finely pulverized coal removes matters from fluids; but Soyka has found that under conditions of sufficient dilution and exposure to air a real destruction of the absorbed organic matter also takes place, so that even substances that are otherwise unchangeable, as, for instance, strychnine, are as completely destroyed by it as if they had been burned in the fire. This explains why the soils of many places that have been inhabited from the earliest times so frequently look to the naked eye, when they are dug over, like virgin soils. Traces of impurity are visible only in places where more is demanded of the ground than it can perform. It is therefore not surprising that the ground under the sewers of Hamburg does not appear to have been contaminated to any great extent. Wolffhügel's investigations, on the other hand, show that the ground under cesspools and drains, in which filth accumulates in a more concentrated form than in flushed sewers, presents a very different appearance. Professor Hofmann's most recent investigations in the soil of the streets of Leipsic show that the ground under the bottom of even badly built sewers is much cleaner than that over their tops or that under the pavements.
Very instructive are the researches of Emmerich, showing that various kinds of foul water, which are sure to kill when injected under the skin of rabbits, become harmless as soon as they are shaken up with common sand, or if they are diluted with a certain proportion of pure water.
Every foul soil which we. cease to pollute purifies itself again in the course of time, and every clean soil, to which no more impurity is introduced than it can work up, remains clean. It is certainly interesting that this purification of the ground is dependent for the most part on the activity of the lowest organisms, as has been proved with reference to the conversion of nitrogenous organic matters in the ground into nitric salts, or to what is called nitrification, by the researches of Schlösing and Müntz. We know as little of the nitrifying germ in the ground as we do of the cholera and typhus germs, but we can and must judge of it by its works. We learn from its operations that the representatives of the lowest organic life, of the cell-life, perform not merely injurious but also very useful functions, that they are not merely noxious or poisonous plants; and we need not be surprised if a later age, when more has been learned in these matters than is now known, shall cultivate the useful bacteria, and make war only upon the injurious ones.