INTRODUCTION. 1 9 



to be found in the soil. This depth, usually reckoned at 

 about twelve feet, varies of course according to the nature 

 of the soil, its moisture, porosity and temperature, and 

 also according to the amount of organic matter that lies on 

 the surface ; cultivated ground always containing more 

 organisms and to a greater depth than fallow ground having 

 the same geological characters. 



The relation of all this to our water supply is obviously 

 one of paramount importance. If water be taken from near 

 the surface of soil in which there is a large quantity of 

 organic matter present, there must necessarily be a large 

 number of putrefactive organisms in it, especially those of 

 an aerobic nature ; if however we take water directly from 

 the deeper layer, these putrefactive organisms are usually 

 absent, but a number of " water " organisms are now present, 

 which under special conditions, especially if the water be 

 kept perfectly quiet and unoxygenated, and a high tempera- 

 ture be maintained, can develop in the water, from which 

 they may in turn be cultivated by certain well-known 

 methods. If water be taken from a much deeper layer, 

 micro-organisms are found to be almost or altogether absent, 

 and not only micro-organisms, but organic matter, although 

 in some cases in which micro-organisms are almost entirely 

 absent, organic matter may still be present in appreciable 

 quantities. The superficial layers of earth in this case act, 

 not only as a mechanical, but also as a biological, filter ; 

 the water, with its contained organic matter, passes through 

 the successive layers in which bacteria can grow, and gradu- 

 ally percolates to those layers of earth where there are no 

 organisms. It has been demonstrated that even deep natural 

 water contains facultative aerobic organisms, unless it is 

 obtained at once after it has undergone natural filtration — 

 that is water that has not stood in an underground basin — 

 when it may be almost germfree. The organisms cannot go 

 down with the water ; first, because they are held back 

 mechanically, the soil acting as a porous filter, by which solid 

 particles, extremely minute as they are, are kept back ; but, in 

 addition, and quite apart from this purely mechanical effect, 

 the bacteria (many of which are unable to develop without 

 oxygen) cannot leave the surface with impunity, and such as 

 are carried down by the action of the water die as their 

 supply of oxygen is gradually cut off ; for, in consequence 



