[82] This very important influence, exerted by the earth on various organic infiltrations, is referred to in the text only under one point of view; only as it occasions the deterioration of land-springs in urban districts, and renders their water unfit for consumption. But the subject has another equally important side. Such springs, having their waters laden with nitrates, represent the continuous removal of organic impurities which otherwise would contaminate the air. The evil of spoiled springs, therefore--while it necessitates for every urban population that their water-supply shall be artificially furnished from a distance, has great countervailing advantages. A given organic soakage will cease to vitiate the atmosphere by evaporation, in proportion as it gravitates to lower levels, and undergoes those chemical changes which accompany filtration through the soil. Hence it is evident that, for the healthiness of inhabited districts (where extensive soakage of organic matters is almost invariable) it becomes most important to maintain, or by artificial measures to accelerate, this down-draught through the soil; and the reader will scarcely need to be reminded, that, in those improvements of metropolitan sewerage, which it is a chief object of this Report to advocate, complete provision for the continuous drainage of soil is implied as an essential part. [83] For a fact strikingly illustrative of this, I am indebted to my colleague, Dr. R. D. THOMSON, Lecturer on Chemistry at St. Thomas’s Hospital. At Liverpool--in three wells which he examined, distant severally 760, 800, and 1050 yards from the Mersey, he found the water brackish from marine soakage, containing four or five hundred grains of solid matter _per_ gallon, and totally unfit for consumption. In final reference to the quality of water, whether supplied by our trading companies or derived from springs within the City, I think it expedient to mention that, against its lesser impurities, great protection is given by filtration through animal charcoal, as in various ‘filters and purifiers’ which are before the public. These protective means do not lie within reach of the poorer classes; nor, whatever their accessibility to individuals, can any such personal arrangements render it less important to provide that water--the first necessary of life--be supplied for universal use in its utmost procurable purity. * * * * *