These remarks must suffice for the more historic portion of the subject, viz. the progress of sewage purification during the last century, and it is time now to direct attention first to natural and afterwards to artificial sewage treatments.
[4] Two of these have since retired.
[5] This report has just been issued (August 18, 1902), and although the special reports it contains are of the greatest interest, it is not necessary to refer to it again in these observations.
[6] The expression “subsidence tanks” is intended to denote tanks which are used in such way that little or no septic action is produced.
III. THE SUBSOIL.
General remarks on subsoil and its properties.
Before dealing more in detail with the processes taking place in the pores of the subsoil of sewage farms, it may not be out of place to make here a few general observations on the mechanical structure of soil, its permeability, water capacity, retentive power, the capillary movements in the same, its temperature, the subsoil air, the movement of water in and through the same, the micro-organic life in soil, and its absorbing powers.
1. Mechanical Structure of Soil.
Size of grain and pores.
Here is of interest the size of the grains or particles composing the soil, the size of the pores and their collective capacity.