Introductory. 



follow the results of the working of the farm to 

 the best advantage. It is probable that attention 

 to this point would do much to foster the desire 

 on their part to turn out the best effluent pos- 

 sible, at the expense, if necessary, of the crops. 

 This question is raised quite apart from the larger 

 one of appointing a qualified chemist in connec- 

 tion with all large sewage disposal works. In the 

 case of a new farm it would seem advisable, if 

 practicable, that the prospective manager should 

 be on the spot while the works were being carried 

 out, as he would thereby obtain an insight into 

 details which otherwise it might take him some 

 time to discover (e.g., the nature of the soil and 

 subsoil on different parts of the farm, as disclosed 

 by drainage operations). In connection with this, 

 it may be remarked that the soil and subsoil are 

 rarely uniform in nature throughout a farm, and 

 that therefore the various plots cannot all take 

 the same quantity of sewage. We are unable to 

 recommend the abandonment of farming operations 

 even in connection with filtration sewage farms, 

 because, if intelligently pursued, they make for 

 profit with increased efficiency of the land. The 

 farming operations, however, should always be 

 under the control of the authorities responsible for 

 the proper working of the farm, and the man- 

 ager should receive written and explicit directions 

 to regard the crops as of secondary importance to 

 the uniform and satisfactory purification of the 

 sewage." (Part I., pages 115 and 116.) 



Meohanioal 

 Analysis. 



" As was recognised thirty 

 years ago by the Rivers Pollu- 

 tion Commission, the purifica- 

 tion of sewage by filtration through a soil depends 

 largely upon the physical structure of the latter, 

 as well as upon its depth. It was therefore thought 

 advisable to make a careful mechanical analysis 

 of the soils of those sewage farms which have been 

 under prolonged observation. In the case of stiff 

 clay soils, through which little filtration can take 

 place, a mechanical analysis has, of course, less 

 value than in the case of porous soils. Some of 

 the main points in connection with the filtration 

 of water through soil are admirably summarised 

 by Whitney in the U.S. Weather Bureau Bulletin 

 No. 4, as follows : ' The circulation of water in the 

 soil is due to gravity, or the weight of water, 

 acting with a constant force to pull the water 



downward, and also to surface tension, or the con- 

 tracting power of the free surface of water (water- 

 air surface), which tends to move the water either 

 up or down, or in any direction, according to cir- 

 cumstances. There is a large amount of space be- 

 tween the grains in all soils in which water may 

 be held, ranging from about 30 per cent, in light 

 sandy lands to 65 or 70 per cent, in stiff clay 

 soils. The relative rate of movement of water 

 through a given depth of soil will depend upon 

 how much space there is in the soil, upon how 

 much this space is divided up — i.e., upon how many 

 grains (particles) there are per unit volume of soil, 

 upon the arrangement of the grains of sand 

 and clay, and upon how this skeleton structure 

 is filled in and modified with organic matter. 

 It also appears that the ordinary manures 

 and fertilisers change this surface tension or 

 pulling power of water; that they also change 

 the arrangement of the grains, and conse- 

 quently the texture or structure of the soil may 

 be changed and the relation of the soil to water, 

 through the effect of the ordinary manures and 

 fertilisers in causing flocculation or the reverse. ' 

 The volume occupied by any given weight of a 

 soil is only partly filled up by solid particles, there 

 being small air spaces between these. Owing, 

 however, to the vastly greater number of particles 

 in heavy soils, their individual air spaces are very 

 much smaller than the individual air spaces of a 

 sandy or a gravelly soil. Filtration therefore pro- 

 ceeds with much greater difficulty in the case of 

 clay, owing to increased friction. It can, of course, 

 be seen at a glance that, in a given volume of a 

 coarse gravelly or sandy soil, there must be a 

 smaller number of particles than in an equal 

 volume of a light loam, and similarly it is evident 

 that a light loam contains fewer particles than 

 a heavy loam or a clay. Experience has proved 

 that a light loam gives better results, as regards 

 sewage purification, than either sand on the one 

 hand or heavy soil on the other, but it was with 

 the hope of finding a more or less definite relation 

 between the number and size of the particles in a 

 soil and the purifying powers of that soil that this 

 investigation was undertaken; or, to put it differ- 

 ently, one object of this investigation was to at- 

 tempt to find an approximate limit — as regards 

 efficient filtration of sewage— for the number of 

 particles permissible in unit volume of a soil. The 



