Further--as involving an important sanitary principle, I would say, that the great object which must be aimed at is not the mere chemical neutralisation of certain stinks which arise within your jurisdiction, but the closest possible limitation, and the promptest possible removal of all those materials which are decomposed into fœtid products. Admirable, no doubt, is that arrangement by which Nature, stationing a sense of smell at the inlet of our breath, cautions us by this vigilant sentinel against the inhalation of many poisonous airs; but, in respect of organic decomposition, I am in no degree satisfied that its odorous products are its only, if even its principal, agents of injury; nor have I any reason to suppose that the real detriment to health which arises from breathing the miasms of sewers or marshes, of cesspools, burial-grounds, or slaughter-houses, would in any important degree be lessened by the mere mitigation of fœtor in their effluvia. Offensive as these are, they at least answer the useful purpose of warning us against the other poisons with which they are associated.
Let me likewise take the opportunity of correcting a misapprehension, which, by the use of an inappropriate word, is sometimes shown to exist on this subject. The agents in question are spoken of as _dis-infectant_. As there is no scientific reason whatever for believing that they in any degree interfere with the spread of epidemic or infectious disease, and as an erroneous opinion on this point may lead to the neglect of measures which are truly precautionary and useful, I think it well to state explicitly, for your information, that I have no evidence of their possessing any other utility, in the respects under consideration, than simply and singly that of removing stink from the atmosphere around them.
For reducing to a _minimum_ the exhalations which arise from sewers and house-drains, it appears to me that the following are the essential principles: First, to render the current through them as rapid as possible; and, above all, by every care for their form, their junctions, their slope, and their material, to provide against the occurrence of obstructions and deposit: Secondly, to employ in their construction, so far as may be possible, such substances as are porous in the least procurable degree; such as consequently will be least apt to imbibe and retain in their interstices any considerable impregnation from the fœtid fluids running over them at intervals; such, too, as will be least likely to permit soakage into the surrounding soil: Thirdly, by reducing the size of drains and sewers to the lowest dimensions compatible with a full performance of their uses, to diminish to the utmost the extent of their interior evaporating surface, and of those large chambers which they now offer for the evolution, retention, and diffusion of gases.