486 Town Sewage. 



It is important to remark tliat tlie drainage from the more 

 porous and less naturally fertile soil of the five-acre field (which, 

 however, gave the largest increase for a given amount of sewage) 

 contained less of almost every constituent enumerated than the 

 more argillaceous and more naturally fertile soil of the more 

 steeply sloping ten-acre field. The result is more particularly 

 marked in the case of ammonia. 



This and other results of common experience tend to show that 

 a soil which may contain a comparatively small proportion of 

 clay, but which is thoroughly porous, is as a rule much better 

 adapted for sewage irrigation, both as regards the utilisation and 

 the purification of the sewage, than richer, stronger, but less 

 permeable land. 



The next Table has reference to samples taken in another field 

 at Rugby during very dry weather in the summer of 1864. The 

 soil here was light and gravelly, the subsoil gravelly, but (as 

 Table IX. shows) it had done the work of absorption as well, if 

 not better than the other fields. (See following page.) 



It had been intended to take samples under various conditions 

 of the weather ; but the continuous drought of 1864 prevented 

 this being done. The plan of collection was to take of sewage 

 about a gallon, and of drainage about half a gallon, eight or ten 

 times during the ten or twelve working hours of the day ; at the 

 end of the day, after well shaking, to take a gallon from such 

 mixture, and to repeat this for six consecutive days until six 

 gallons of each were obtained, when, after well shaking, a two- 

 gallon sample of each was bottled off for the purpose of analysis. 



In judging of all these results it must be borne in mind that, 

 except when the land is already saturated Avith water, a gallon of 

 drainage will represent much more than a gallon of sewage, and 

 its sewage-constituents must have been derived from more than a 

 gallon of sewage. The non-retention of manurial matter by the 

 soil is, therefore, less than might seem at first sight. 



As in the earlier analyses, so in these the quantity of matter 

 in suspension in the drainage was very small, and being obviously 

 in great part derived from the soil, it was not submitted to 

 quantitative analysis. 



A considerable proportion of the phosphoric acid of the sewage 

 was in suspension ; but there was none of it in suspension in the 

 drainage. 



Of the inorganic constituents in solution, by far the larger pro- 

 portion of those which are most likely to become deficient for a 

 crop was retained by the soil. Thus smaller proportions of both 

 the potash and the phosphoric acid passed off in the drainage than 

 of any other constituents. Soda was also retained to a consider- 

 able extent, magnesia in a less degree, and lime less still. Of 



lime. 



