To meet these evils, several methods have been suggested for precipitating the valuable portions of the sewage, and preparing them in a dry state for the purposes of commerce, during which process the matters are also deodorized and disinfected. It is a matter of dispute, indeed, whether the portions of sewage which are specially serviceable for agricul tural purposes can he preserved in a dry form ; but besides the evidence resulting from experiments with this form of manure, we have the favourable testimony of Liebig, who thus writes :—" Gypsum, chloride of calcium, sulphuric or muriatic acid, and super-phosphates of lime, are substances of a very low price ; and if they were added to urine until the latter lost its alkalinity, the ammonia would be converted into salts, which would have no further tendency to volatilize. When a basin, filled with concentrated muriatic acid. is placed in a common necessary, so that its surfhee is in free communication with the vapours arising from below, it becomes filled, after a few days, with crystals of muriate of ammonia. The ammonia, the presence of which the organs of smell amply testify, combines with the muriatie acid, and loses entirely its volatility, and thick clouds or finnes of salt hang over the basin." And in the quotation which we have previously quoted, the same chemist states his opinion. that the moisture may be removed without permitting the e-“..ape of ammonia, and the sewage put into such a form as to allow it to be transported to great distances. In opposition to this, some persons have asserted, that where the smell dues not exist, the fertilizing property has been lost ; and others, that although the offensive odour is got rid of, the deleterious properties still remain. These opinions are indeed destruc tive of each other, and we are inclined to yield the greater amount of credit to Liebig's testimony. Mr. Richard Dover, who propounded a scheme for disinfecting the London sew age, and preparing it for transport in a dry state, made some experiments upon sewage-water taken from the Northumber land-street outlet, in the presence of several persons corn irtent to form a judgment on the subject, and who testify to having been satisfied as to the success of the experiment. Manure prepared by this process has been experimentally applied to agricultural purposes, and, it is said, with very satisfactory results. Mr. Dover further proposes to apply the water, which is left, for the purpose of bleaching and other commercial undertakings.
Such are the statements with regard to the employment of sewage-matters for agricultural purposes : that they are useful fertilizing agents, there seems to remain no reason to doubt ; but there still does remain a question as to whether it can be profitably employed for this purpose, considering the subject in the light of a commercial undertaking. Can it compete with other manures already in the market, or will the expense of its manipulation destroy its value as a market able commodity. These questions still remain to be con sidered ; for although the evidence which does exist seems to favour the speculation, yet this is not sufficiently decisive to justify an undertaking of the kind on a large scale.
Now the very first principles of drainage depend upon the resolution of such doubts ; for if the sewage-matter is to be thus employed, we should adopt a very different system to that which would be adopted, had we solely to get rid of the noxious refuse. There would be also a difference of treat ment, according to the condition in which the manure was to be applied, whether in a wet or dry state ; for in the one case we should be tied down to the most economical method of distributing it over the surrounding country by means of pipes, and should have to regulate our levels and outlets accordingly ; whereas, under the latter conditions, we should be left at much greater liberty in arranging such prelimi navies.
In the first place, if we treat sewage matters as only deleterious to public health, and incapable of being turned to any useful account, we have to deal with a very plain and simple question ; our sole object is to get rid of it. Simple, however, as may be the object to be attained, it is by no means easy of attainment ; and of the two cases, this is pro bably the most difficult. We are well aware that we cannot annihilate the elements of which the matters are composed, however much we may be able to alter their combinations, and perhaps destroy their characteristic qualities. Granting, then, that the noxious properties of the soil can be removed, we find two courses open to us, either to disinfect it, and then dispose of it in any manner most suitable to the locality, or else to remove it, in its primary state, to a distance front hu man habitation. if we adopt the former course, we may dis infect it either during its passage to its outlet. or may collect the various streams into one or more reservoirs, as the nature of the locality may render advisable, and there disinfect it; after which there will probably be no difficulty of disposing of' it, either by conducting it into a river or running-stream, or by some other means which the nature of the place may suggest. If the latter expedient be resorted to, we shall have much greater difficulties to encounter, for we can scarcely consider it inoperative for evil, until it is safely and fairly launched into the wide ocean ; until it has arrived at this point, we are not secure against its fatal influence. This, however, is no easy matter to accomplish: to effect the object in view, we instinctively look for some natural channel, by means of which it may be conveyed to its proposed destina tion, and this is presented to us in the shape of is stream or river ; to conduct it to the sea by artificial means, would, in the majority of cases, be entirely out of the question, on account of its enormous expense; we must therefore adopt the river as a last resource. We are well aware that this is objectionable on many accounts ; but if we are compelled to get rid of the offensive matter, we would ask, what other method of ellectin,ff this object remains open to us. We know the question will arise, as to whether under such circumstances the sewage ever reaches its destination ; and it would be very difficult to answer positively that it does, and therefore pro bably we must rest content with a palliative measure, and remove it from the more densely populated districts only. Where there is a good river, not a tidal river, this may be effected readily, fur the sewage discharged into the river will be conveyed with its waters in an onward course without interruption, depositing probably some of its heavier pa•ti cles, but not in such large quantities as to become dangerous. In a tidal river, however, the state of the case is altered, for although we have the advantage of the ebb-tide in conveying the refuse towards the sea, we have the corresponding dis advantage of the flow bringing it back again.