1889-90.] NATURAL HISTORY OF GROUND WATERS. 155 



eight to eighteen hours, while clayey matters in suspension often take 

 one week to subside. The sources of these impurities are those common 

 to all river waters, viz., those due to vegetable matter from streams and 

 rivulets rising and bearing with them the soluble matters from the bogs 

 and virgin soils whence they spring, as also in certain localities the sew- 

 age deposits which may be poured in from towns along their banks. 

 From such causes is due the fact that river waters are commonly con-, 

 sidered unsafe sources of public water. They not only are readily con- 

 taminated, often to an extraordinary degree, but the swiftness of their cur- 

 rents often bears these impurities onward before the purifying influences 

 have time to perform their work, and hence sedimentation and filtration 

 beds have been brought into requisition in the endeavor to bring the 

 water into a potable condition. 



We see in these operations going on before our eyes in river waters an 

 illustration of what goes on in the purification of ground-waters, only 

 that the stages are in the latter instance more slowly and much more per- 

 fectly performed. The rain which falls on the surface is absorbed by the 

 ground and bears with it minute amounts of carbonic acid and ammonia 

 due to the combustion of the organic matters of the soil. These gaseous 

 impurities of the air, which have had their origin in the ground, become 

 of much value in aiding the disintegration of the soil and thereby promote 

 the percolation of water through its layers. The rains receive, however, 

 from the upper layers, which always contain organic matter either of 

 vegetable or animal origin, soluble materials such as the above, soluble 

 albumen, and other products of the humus, etc., and moreover what in 

 addition to them has been borne to the earth, living organisms of various 

 species, notably of the bacteria which, as is now well determined, are the 

 agents of the decomposition of organic compounds, and these are in the 

 soil in innumerable amounts. It has been already stated that carbonic 

 acid and ammonia are the soluble compounds resulting from the decompo- 

 sition of the organic matters of the soil. Now we find that wherever 

 albumenoid matters, especially, are present in soils or in cultures which 

 are artificially prepared, causing the multiplication therein of bacteria, 

 these two gases are amongst the principal products. Subsequent chemical 

 action as by oxygen, forms carbonates while nitrates develop out of the 

 ammonia present. Manifestly, however, this extent of the oxidising pro- 

 cess depends upon the depth of the organic materials in the soil, its por- 

 osity as regards movement in it of ground air and the depth to which the 

 bacteria are found to have extended. Now while the first two conditions 

 are interdependent and to some extent condition the depth at which bac- 

 teria are to be found, they show us that there are several other factors 

 in the problem which must be considered. 



