cially fatal, and spread with the greatest virulence, in densely populated and badly ventilated districts, and it is in these "fever-nests" that epidemic diseases, which prevail during certain septic conditions of the atmosphere, are attended with the highest mortality and the greatest sick-rate.

Of other diseases developed by respired air, there can be no question that phthisis pulmonalis holds a prominent place on the list. A large mass of evidence has been collected from various sources bearing on this point, but the fact is now so fully recognised by the medical profession generally that a few instances will suffice. In the celebrated report of the Army Sanitary Commission, published in 1858, it was proved beyond all doubt that the excessive mortality from consumption amongst soldiers, and in particular regiments, was due to overcrowding and insufficient. ventilation. Previous to that inquiry, the cubic space per soldier in the barracks of the Foot Guards only amounted to 331 cubic feet, and the phthisis mortality was as high as 13.8 per 1000. In the Horse Guards, on the other hand, with a space per man of 572 cubic feet, the mortality from phthisis did not exceed 7.3 per 1000. It was found that phthisis prevailed at all stations, and in the most varied and healthy climates, the vitiated air in the barracks being the only condition common to all of them. In consequence of this excessive mortality, the Commissioners recommended that the cubic space allowed per man in barracks should be increased, and the ventilation improved, with the result that, from the time their recommendations were acted upon, the number of phthisical cases occurring at all these stations has materially diminished.

Similar evidence is afforded by the statistics of the Royal Navy, and notably as regards the civil population, in the Report of the Health of Towns Commission, published in 1844. Indeed, it has been fully established that not only phthisis, but other lung affections, such as pneumonia and bronchitis, are generated to a large extent under like conditions, and the same may be said of such diseases as scrofula, and others of an adynamic type.

When air is vitiated by the exhalations of the sick, as in hospitals, there is a risk of gangrene and erysipelas spreading, especially in the surgical wards. The period of convalescence, in many cases, is retarded, and the mortality rate increased. Pus-cells and putrefying particles are thrown off from purulent discharges, and finding a suitable nidus elsewhere, may communicate a special disease, and thus act as a true contagium. prevalence of purulent ophthalmia, under certain conditions, and the spread of lung-disease in badly ventilated ships, when the disease appeared to be propagated from person to person, can only be fully explained on some such theory as this.-(Parkes.)

The

2. Air rendered Impure by Sewage and Cesspool Effluvia.-Amongst the gases generated by the decomposition of fæcal matter, whether occurring in sewers or cesspools, may be enumerated, carbonic acid, nitrogen, sulphuretted hydrogen, light carburetted hydrogen, and ammonium sulphide. Dr. Letheby found that sewagewater, excluded from air, and containing 128 grs. of organic matter per gallon, yielded 1.2 cubic inches of gas per hour during a period of nine weeks. But the amount of gaseous products given off under ordinary circumstances must vary greatly, according to the

dilution of the sewage, the rapidity of flow, temperature, ventilation of the sewers, etc. In comparing the results of analyses made by various chemists, it would appear that the oxygen is diminished, and the carbonic acid greatly increased, but that sulphuretted hydrogen and ammonium sulphide, when present, exist only in very small quantities. The peculiarly fœtid smell of sewage-gas is therefore owing to the presence of organic matter, whose exact chemical composition, however, has not been determined. Dr. Odling believes it to be carbo-ammoniacal. It is alkaline in reaction, and speedily decolorises solutions of potassium permanganate. Like other organic effluvia, it promotes the growth of fungi, renders milk sour, and taints meat.

It is doubtful whether the effects of sewer-air upon the health of men employed at work in sewers can be said to be very injurious. Indeed, the researches of Dr. Guy and Parent du Chatelet, at first sight, go to prove that this class of labourers enjoy a marked immunity from diseases which can be attributed to seweremanations; but, as has been shown by Dr. Murchison, there are several elements of error in their statistics which mar their conclusions. For example, Dr. Guy's researches were made before enteric and typhus fever were fully recognised as distinct diseases, and Parent du Chatelet's statistics were not only too scanty for a fair deduction, but the majority of the sewer-men whom he examined had been employed at that special work for only a short period. According to Dr. Murchison's experience, enteric fever is by no means uncommon among these men, and Dr. Peacock's inquiries led him to express a similar opinion. But whatever the issue of this question, it seems to be quite certain that con

stant exposure to sewer-gases diminishes the risk of being injured by them. A remarkable instance of this apparent immunity enjoyed by workmen, and the disastrous effects upon those whose exposure to such gases was only casual, is afforded by an event that occurred at Clapham in the autumn of 1829-20 out of 22 boys at the same school were seized with violent vomiting, purging, prostration, and fever, within three hours. One boy had been seized with similar symptoms two days before, and died; another also succumbed. So alarming was the outbreak that poisoning was suspected, but, after careful investigation, it was found that the sole cause of disease was to be attributed to the opening of a drain at the back of the house. This drain had been choked up for many years, and had been opened two days before the first illness occurred. The effluvia from the drain were most offensive, and the boys had watched the workmen cleaning it out; none of the workmen, however, were subsequently attacked with any of the symptoms which so seriously affected the boys.—(Murchison.)

A similar instance, with more disastrous results, is recorded as having taken place only a short time ago. All the hands employed in the winding-room of one of the mills at Swillbrook, Preston, were suddenly seized with symptoms of alarming illness. At first it was supposed that the illness was caused by minute particles of dye thrown off by some coloured yarns, but it was soon discovered that the air of the apartment was contaminated with mephitic gases issuing from a drain which emptied itself into an adjoining water-course. The drain had become blocked up in consequence of a "fresh" in the river, arising from heavy rains, and in

this way the sewage was allowed to accumlate. Several of those who were attacked died, and others were seriously ill for some time.-(From Food, Water, and Air.)

While numerous other instances are recorded of the evil effects of the air of sewers, cesspits, drains, etc., in producing temporary ailments, such as nausea, vomiting, diarrhoea, and headache, the great interest which attaches to this inportant subject rests on the development and spread of enteric fever. Dr. Murchison, in summing up his evidence with regard to the spontaneous origin of this disease, says, "I readily admit that we cannot succeed in tracing every case of enteric fever to organic impurities. But if the disease can be traced to such causes in a few undoubted instances, it is reasonable to infer that its causes are similar in all cases where it has a spontaneous origin. As already stated, the actual poison may, like the miasmata which give rise to ague, be inappreciable to the senses, or by chemical research. During the last four years (1858-1862), however, I have met with few examples of enteric fever which, on investigation, I could not trace to defective drainage, the existence of which was occasionally unknown to the inhabitants of the infected locality."

More recent researches tend to show that enteric fever is perhaps seldom generated de novo, but that it is essentially a specific and infectious disease. No doubt, a great deal of the discrepancy of opinion which has hitherto existed, and still exists, concerning its etiology, is due to the difficulty of diagnosing the many different varieties of the disease. The point, however, which has especially to be borne in mind is this, that sewers often become the real channels by which the contagium is