I shall, first, briefly consider what is meant by the term "sewer gas." This term, as Prof. W. Ripley Nichols has truly said,* is "an unfortunate one, and gives rise to a quite widespread but very erroneous idea. Many seem to suppose the 'sewer gas' to be a distinct gaseous substance, which is possessed of marked distinguishing characteristics, which fills the ordinary sewers and connecting drains, and which, as a tangible something, finds its way through any opening made by chance or by intention, and then, and only then, mixes with the atmospheric air."

Sewer gas is a mechanical mixture of a number of well known gases, having their origin in the decomposition of animal or vegetable matter, with atmospheric air. This mixture is continually varying, according to the more or less advanced stage of putrefaction of the foul matters, which form a sediment and a slimy coating of the inner surfaces in drains and pipes. It is also variable with the character of this sediment or deposit, and with the physical conditions (moisture, heat, etc.) under which the decomposition takes place.

* See Prof. W. Ripley Nichols' report upon chemical examination of the air of the Berkeley street sewer, in Boston, Mass., 1878.

The principal gases found in sewers and drains are oxygen, nitrogen, carbonic dioxide, carbonic oxide, ammonia, carbonate of ammonia, sulphide of ammonium, sulphuretted hydrogen and marsh gas.

The three first-named gases are the principal constituents of the atmosphere, surrounding the globe, and are found present in the following average proportion, viz.:

20.9 vols. oxygen 79.l vols. nitrogen in 100 vols, of air, together with 2 to 5 vols. carbonic dioxide in 10.000 vols. of air.

According to R. Angus Smith the amount of oxygen is:

In the average, 20.96 vols. in 100 vols. of air. In pure mountain air, 20.98 vols. in 100 vols. of air. At the sea shore, 20.999 vols. in 100 vols. of air. In streets of populous cities, 20.87 to 20.90 vols. in 100 vols. of air.

The air in sewers and drains contains much less oxygen, as some of it combines with the carbon of putrefying organic matter forming carbonic dioxide. The amount of nitrogen in the air of sewers is little different from that in the atmosphere which we breathe; but the amount of carbonic dioxide present is greatly increased. .

The lowest amount of oxygen in sewer air is recorded to be 17.4 vols. in 100 vols. of air; the amount of carbonic dioxide is in the average 2.3 vols. in 100 vols. Sulphuretted hydrogen varies greatly, but the quantity is generally so small as not to be easily determined. Still more difficult is it to find by chemical analysis the proportion of other gases of decay.

In well ventilated and well flushed sewers, Dr. Russell, of Glasgow, found the following ratio:

20.70 vols. of oxygen in 100 vols. of air. 78.79 vols. of nitrogen in 100 vols. of air. 0.51 vols. of carbonic dioxide in 100 vols. of air. No sulphuretted hydrogen in 100 vols. of air. Traces of ammonia in 100 vols. of air.

Carbonic oxide is present only in excessively minute quantities, and even then it it may have entered the sewer or drain through leakage of illuminating gas from gas mains.

In the absence of more satisfactory methods of analysis, it is usual with ■chemists to determine the amount of pollution of the air, or the organic matter in it, by determining the amount of carbonic dioxide present) assuming that there is a certain fixed proportion between the amount of carbonic dioxide and the organic matter.* Thus, Prof. W. Ripley Nichols records as the average of many carefully conducted experiments in Boston, the amount of carbonic dioxide in a sewer in that city as follows:

The average of 31 determinations in January, 1878, was 8.7 vols. of Co2 in 10,000 vols. of air. 44 determinations in February, 1878, was 8.2 vols. of Co2 in 10,000 vols. of air. 47 determinations in March, 1878, was 11.5 vols. of Co2 in 10,000 vols. of air. 12 determinations in April, 1878, was 10.7 vols.

of Co2 in 10,000 vols. of air. 8 determinations in June, 1878, was 27.5 vols.

of Co2 in 10,000 vols of air. 8 determinations in July, 1878, was 21.9 vols.

of Co2 in 10,000 vols. of air.

6 determinations in August, 1878, was 23.9 vols. of Co2 in 10,000 vols. of air.

7 determinations in January, 1879, was 8.0 vols. of Co2 in 10,000 vols. of air.

* Such is strictly true only for air fouled by respiration, while it may not give accurate results in other cases.

In regard to this interesting question I must refer to the Report of Prof. Ira Remsen on the subject of organic matter in the air, published in the National Board of Health Bulletin, vol. 2, No. 11.

14 determinations in February, 1879, was 11.6 vols. of Co2 in 10,000 vols. of air.

20 determinations in March, 1879, was 11.8 vols. of Co2 in 10,000 vols. of air.

He remarks: "It appears from these examinations that in such a sewer as the one in Berkeley street, which, being of necessity tide-locked, is an example of the worst type of construction, the air does not differ from the normal standard as much as many, no doubt, suppose. In a general way, as we have seen, there is a larger amount of variation from normal air during the warmer season of the year: but even When the amount of carbonic acid was largest, it was only ex-tremely seldom that sulphuretted hydrogen could be detected." . . . . "I think it should be said that the soil pipes and house drains are much more likely causes of discomfort and danger than the sewers."

Hence the importance of a thorough Ventilation of all the soil, waste and drain pipes in a building.

Are the above-named constituents sewer air the origin or cause of the sickness so commonly attributed to the inhaling of sewer gas ?

Although many of the gases named are poisonous, if inhaled into the system in large quantities, and may, even if present in smaller quantity, cause nausea, asphyxia, headache, vomiting, etc., none of them can be said to produce any of the so-called "filth diseases." To determine the exact origin of these is a still unsolved problem of physiology. While some believe that the particles of decomposing organic matter, present in sewer air and known as "organic vapor" cause disease, others seek the origin of the latter in microscopic spores or germs which live and feed upon such organic vapor and are capable of reproduction under favorable conditions, such as presence of putrefying filth, excess of moisture, heat, lack of oxygen, etc.

Whatever theory may be accepted as true, it is evident that, by preventing the decay of organic matter within sewers, drains and soil pipes, or by depriving these germs (if such be the cause of disease) of the conditions facilitating their reproduction, we can best prevent the outbreak of excremental diseases. In other words, by completely removing ax speedily as possible all waste matters from the dwelling by pipes thoroughly and tightly jointed, and by a sufficient dilution of the air in these pipes with oxygen, the danger of infection, arising from defective drainage and plumbing, may be reduced to a minimum.

It should be mentioned that some hy-gienists, notably Dr. Soyka and Dr. Renk, both assistants of Pettenkofer in Munich, have lately denied the existence of any positive proof of a connection between sewer gas and the spread of epidemic diseases - just as Naegeli and Emmerich doubt the possibility of infection from drinking water contaminated by sewage. Dr. Renk considers the exclusion of gases of decay from the interior of dwellings necessary only so far as they are offensive to the sense of smell. In this view, however, I cannot concur; in regard to "filth-diseases," their causes and origin, I accept the theory of Dr. Simon, Parkes and others.