T^§\ 'TRANSACTIONS iSgg-'oO 



surface of the ground, flows along this surface, or soaks into it, 

 taking momentarily into solution more and more of the soluble 

 matters with which it comes into contact. These are partly 

 inorganic salts, chlorides, sulphates, carbonates, silicates, etc. — 

 and partly organic matters of more or less complex nature, the 

 products of the decay of vegetable and animal matter. If, for 

 example, in Fig. 3, the region 2 to 3 is a cultivated farming 

 country, somewhat sparsely inhabited, the organic impurities 

 will chiefly be of a vegetable kind — rotting vegetation, the 

 manure of the fields, etc. ; but if it be a village or town, the 

 organic matters will be more largely of animal, and especially of 

 human origin. To these waste products the term sewage is 

 properly applied. The ground-water of this region may be, 

 therefore, much less pure than was the rain water that fell on 

 the surface or gathering-ground, I say 7fiay be, for reasons 

 which will be presently given. 



■ We may dismiss the fw^r^awzV impurities with a word, by 

 Saying that, unless they are present so largely as to give a distinct 

 taste to the ground- water, they are rarely of a kind to be 

 dangerous to health. The organic matters must be mo^ care- 

 fully considered. If they have originated in normal decay, they 

 may be harmless from the point of view of health, even though 

 far from appetizing when we remember their origin ; but if they 

 come from those conditions of decay which we call disease, they 

 may be actively poisonous, and may contain the living germs of 

 specific diseases, such as fevers, diphtheria, cholera, etc. 



Percolation, though a fully aerated soil, has, however, the 

 effect of bringing about purification of such water by the process 

 of oxidation, a process by which organic matter is changed and 

 microbial life destroyed. That this should be effective, the 

 water must filter through several feet of sand or gravel ; and 

 although it is impossible to fix a definite minimum limit to the 

 depth of such a natural filter, it is safe to say that we should 

 insist- on- ten feet, at least, and prefer as much more as we can 

 get;, r . To this end it is necessary that the upper ten feet of the 

 wall of a well should be made quite impervious to water, and 

 the accompanying diagram shows how such a construction 

 can- be brought about. 



r ;. , A well should be so constructed that no water could find 

 (entrance to it without filtration through a depth of soil, at least 



