SANITARY RELATIONS OF THE SOIL. 475 



an organic nature, to so low a degree that they shall he quickly 

 changed and rendered harmless hy the soil. The ground has a certain 

 power to purify itself. 



Falck made the discovery not long ago that water containing in- 

 fectious 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 con- 

 ditions 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. Wolffhiigel'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 differ- 

 ent appearance. Professor Hofmann's most recent investigations in 

 the soil of the streets of Leipsic show that the ground under the bot- 

 tom 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 in- 

 teresting 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 re- 

 searches of Schlosing and Miintz. 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, per- 

 form not merely injurious but also very useful functions, that they 

 are not merely noxious or poisonous plants ; and we need not be sur- 

 prised 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. 



