propagated. The sewer-air, laden with the specific poison, readily finds its way into houses on account of its greater tension, and in consequence of badly-trapped or imperfectly-ventilated drains. It may be inappreciable to the senses, but its baneful effects make themselves felt none the less, and, as recent events have shown, may sometimes exhibit themselves in the most exalted stations of life. Indeed, it would appear that persons of the upper and middle ranks in towns are more liable to be attacked by enteric fever than the poorer classes, and for this reason - the houses of the former are more generally connected with sewers, and, either from structure or situation, are of higher elevation, so that the light sewer gases, in obedience to natural laws, are more apt to accumulate in the drains of such houses, and when the drains are not efficiently trapped or ventilated, to effect an entrance into the houses themselves. Thus it happens that a system of sanitary engineering which is intended to prevent, and does prevent, the development of disease, not unfrequently furnishes the readiest means for its propagation. All this, however, could be frustrated if sewers and drains were always properly ventilated.

One other point connected with the propagation of enteric fever deserves notice :-it seems to be clearly established that the disease may be contracted by inhaling the effluvia from enteric stools previous to their being disposed of, and hence the necessity of disinfecting all such stools so soon as they are passed.(Parkes.)

Amongst other serious consequences of fæcal emanations, the occasional spread of cholera, and the occurrence of autumnal diarrhoea, are specially to be noted.

The outbreak of cholera in the city of London Workhouse, in July 1866, was shown by Mr. Radcliffe (Ninth Report of Medical Officer of the Privy Council) to have taken place, in all probability, in consequence of a sudden efflux of sewer-air from a drain containing choleraic evacuations. Autumnal diarrhoea, again, is found to prevail when the season is warm and dry, and more particularly in badly-sewered districts. In speaking of this subject, Dr. Murchison says, that "circumscribed autumnal epidemics of enteric fever are often preceded by an increase of diarrhoea, and the diarrhoea reaches its acme long before the fever does." After heavy falls of rain the sewers become well flushed, and the diarrhoea subsides.

According to the evidence of Sir Henry de la Beche and Dr. Lyon Playfair, in the Second Report of the Health of Towns Commission, there are strong presumptive grounds for believing that emanations from streams polluted by fæcal matter may be injurious to the health of inhabitants living on their banks. It is stated that many of them were pale, and suffered from dyspepsia, and that cases of fever, when they occurred, were increased in severity. In other instances, however, no such effects have been traced.

When sewage matter is thrown over the ground, or allowed to sink into the soil as in sewage irrigation, the exhaltations given off have likewise been proved to be sometimes productive of serious disease. Thus, Dr. Clouston has recorded an outbreak of dysentery among the patients in the Cumberland and Westmoreland Asylum, which he attributed to the emanations from sewage applied to the land about 300 yards from the asylum. After this outbreak the sewage was allowed to fall into a

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small stream, and for two years the asylum had been free from the disease. At the end of this period, however, the sewage was again applied to the farm, and again the dysentery appeared, although all proper precautions were taken in the way of disinfecting and in applying the sewage. It is to be noted that there was a stiff brick-clay subsoil, and doubtless this prevented the sufficient percolation of the sewage into the ground. (For further observations on this point, see Chapter XII.)

3. Efluvia from decomposing Animal Matter.Under this heading may be included-the effluvia from decomposing carcases; the air of graveyards; and the effluvia from manure, tallow, and bone-burning manufactories.

On almost all these points the evidence is very conflicting. The preponderance of opinion, however, leaves no room for doubt that the effects of all such effluvia upon the health of the general population, when exposed to their influence, are more or less injurious; and in support of this view the following amongst many other confirmatory instances, may be quoted :—

(1.) The effluvia arising from the putrid remains of horses killed on the field of battle have frequently given rise to outbreaks of diarrhoea and dysentery amongst the soldiers. In the French camp, before Sebastopol, when numbers of the bodies of horses lay putrefying and unburied, the effects were so serious that the spread of typhus was supposed to be due to this cause. (Parkes.)

(2.) According to the evidence summed up in the Report on Extramural Sepulture in 1850, the vapours given off from thickly-crowded graveyards, if not actually productive of disease, do certainly increase the sick and death-rate of the immediate neighbourhood.

(3.) Although the health of workmen employed in manure and similar manufactories does not appear to be injured by their occupation, the occasionally disastrous effects upon others, of the effluvia given off, are well illustrated by the following case-In 1847, many of the inmates of Christ Church Workhouse, Spitalfields, were seized with violent attacks of diarrhoea, of an enteric type. It was found that whenever the works were actively carried on, and particularly when the wind blew from that quarter, there ensued an outbreak of diarrhoea in the workhouse. In December of the following year, when cholera was spreading in the neighbourhood, sixty of the children were attacked one morning with violent diarrhoea. In consequence of

this outbreak the owner of the manufactory was obliged to stop work, and the children rapidly recovered. Five months afterwards the works were resumed, and again there was a similar outbreak amongst the inmates occupying the part of the building opposite the manufactory. The works were once more discontinued, and the diarrhoea ceased. (Carpenter.)

The effluvia produced in tallow-making and boneburning, though sometimes very offensive, and therefore an undoubted nuisance in inhabited districts, do not appear to have produced any serious effects which have been recorded. Owing to their being slowly oxidised, such vapours may be detected at very long distances. (Parkes.)

4. Gases and Vapours given off by Alkali Works, Chemical Works, and Brickfields. (1.) The principal gas evolved in alkali works is hydrochloric acid. Its effects on vegetation are so destructive, that an Act was recently passed to ensure more thorough

condensation of the acid gas given off.

This condensation is now carried on with such completeness in most works, that the escaping gases do not cause a turbidity in a solution of silver nitrate, thereby showing that no trace of the acid gas is present. (Roscoe.)

(2.) From chemical works, and especially from those in which gas liquor is utilised for the production of salts of ammonia and other chemical compounds, the injurious gases evolved consist chiefly of sulphuretted hydrogen, ammonium sulphide, and traces of other ammonium compounds. The workmen employed at such works apparently enjoy good health, but when the noxious vapours are not properly consumed by being collected and passed through a furnace, there is no doubt that they do affect the health of the neighbouring inhabitants, though not to any serious extent. In a case tried not long since at Portsmouth, several witnesses gave evidence that they often suffered from nausea, vomiting, and headache, when the wind blew from the direction of chemical works of this description, although some of them lived at over a mile's distance from the works. It is probable, however, that these effects were partly to be attributed to the effluvia of a knackery and manure-manufactory, situated within 300 yards of the works.

(3.) The peculiarly pungent odour of brickfields can be felt at several hundred yards' distance; but though several cases are recorded, in which the existence of a nuisance was fully established, none are quoted as having proved that the health of the neighbourhood was affected.

5. The Air of Marshes.-This generally contains an excess of carbonic acid, light carburetted hydrogen,