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. [Sidenote: 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. [Sidenote: 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. According to the character of the soil, its grains or particles will vary from very large in coarse gravel to very fine in fine sand and clay. [Sidenote: Variable size of pores.] [Sidenote: Surface attraction.] The size of the pores will vary as the size of its grains from large to small, but frequently a certain kind of soil will contain a mixture of large and small pores. The finer the pores the more energetic will, as a rule, be the surface attraction of the grains composing the soil. [Sidenote: Pore-volume.] [Sidenote: With particles of equal size pore-volume amounts to about 38 per cent. of the total space, and sinks down to 10 or 15 per cent. with particles of unequal size.] [Sidenote: With equally sized particles the pore-volume is the same whether the particles are small or large.] The collective capacity of the pores or the pore-volume mainly depends on the equal or unequal sizes of the particles. When the same are of equal size the pore-volume amounts to about 38 per cent. of the total space occupied by the soil, but when this is not the case it may sink to as low as from 10 to 15 per cent. of this space. With equally sized particles the pore-volume is the same whether the individual particles are large or small. In nature it will be the exception to find all the particles of equal size, such a condition of things prevails only when careful sorting by sifting or riddling has taken place, and in the majority of cases the larger pores will be partly filled up by the smaller particles of the soil. 2. PERMEABILITY OF SOIL.