The methods for the study of micro-organic life in air are of comparatively recent date and are daily being improved. According to Roechling about six sets of investigations into the bacterial flora of sewer air have within recent years been made, and from them in complete corroboration of other researches made before by less perfect methods and apparatus. They show that sewer air contains few germs as compared with outer air, sewer air containing on the average from 2 to 9 germs where outer air contained 15 per quart. These germs are related to the germs of the outer air, but not to the sewage itself, which contain an enormous number of germs, a quart containing sometimes as many as 5 billion. Disease germs are not found in sewers with the single exception of the germ of suppuration (staphylococus pyogenes aureus) and up to the present time* only one of the many observers (Uffel-mann) has been able to discover even this pathogenic germ in sewer air. Splashing of sewage, as when a branch sewer enters another or a main sewer with a high fall, may cause a fine state of division of sewage in virtue of which germs may be carried some distance through the air, "even 50 or 60 yards" according to Laws, from which our remedy lies in so arranging the connections as to avoid splashing.

Micro Organisms In Sewer Air 53

*This was written before the experiments of Major Horrocks had been made.

If the sewage falls against the inner curved surface of the drain it may be arranged to do this without splashing even from a considerable height. The experiments of Laws showed that a branch drain emptying its sewage into an egg-shaped sewer 11 ft. high by 9 ft. wide from about the middle of its height produced practically no effect upon the number of micro-organisms in the sewer air.

The experiments of Laws in London and Ficker in Bres-lau seem to show that germs cannot be given off from the slimy skin which forms on the inner surface of sewers on account of the dampness of the air in them. Mr. Laws says in regard to this: "It is really remarkable to find that no organisms are given off from the walls of a sewer which has been empty and open to the air at both ends for such a lengthened period as 12 days. The sewage with which the sewer had been kept full for several periods of 24 hours would contain no less than three to four millions of organisms per cubic centimeter (about 3/8 in. cube) and immense numbers of these must of necessity have been clinging to the walls of the sewer. , . . The velocity of the air current used in the above experiments was 5 ft. and 15 ft. per second, respectively, the latter being far in excess of any current that would normally obtain in a sewer."

Ficker found that a current of air of several yards a second could not lift up germs from a half moist soil, nor even germs which had dried on several substances and adhered to them.

Hesse experimented to see how far germs could be carried in air currents in pipes and sewers before falling against and adhering to their sides. In one experiment with a 2-in. pipe a yard long, coated with nutritive gelatine, he found that the air current deposited a large number of bacteria on the first quarter, less on the second, still less on the third, none at all on the last quarter. Ficker experimented in this line with 4-inch pipes and found the germs were carried as much as 23 feet.

Disease germs find the conditions in the sewers unfavorable to their life and propagation. They cannot survive against the myriads of other germs that crowd the sewage and in their slow death in fighting against these they gradually lose their virulence or power for mischief long before their actual death takes place.

Kirchner says: "We are entitled to say with a probability bordering on certainty that presumably pathogenic germs will never be found in sewer air."

Messrs. Laws and Andrewes went so far as to classify elaborately and comparatively the various kinds of germs found in the outer air, in sewer air and in the sewage itself, and they found the same kinds in sewer air as in the outer air, but a totally different kind in the sewage. They even go so far as to say that so far as they are aware, not a single colony of any of the many species which they found predominate in sewage has been isolated from sewer air.

These investigators even assert that "moderate splashing carried out so as to imitate the inflow of a lateral drain or house sewer produces no variation in the sewer air even within such a short radius as four feet from the disturbance." In view, however, of the absence, still, of accord on this point among all investigators, it is well to avoid such splashing as far as possible.

As for the typhoid fever germ, how very difficult it is to catch it, even in sewage, is clear, says Mr. Roechling, from the report of Messrs. Laws and Andrewes. Although they used the greatest care, they were not able to find this germ once, in ordinary London sewage. Even in the typhoid fever hospital drain inside the hospital grounds, they were able to find only two colonies, and in this drain a quarter of a mile away not a single typhoid germ was found. They probably were killed by other bacteria who were their enemies and vastly exceeded them in numbers; for many harmless germs thrive in sewage, as disease germs appear not to.

Accordingly from all these investigations we may conclude that the cases of typhoid fever and other diseases to which we have alluded were not due to disease germs entering the houses in the air of the sewers, but to contamination of the food or water supply by sewage polluted water, or by insects, especially house flies, passing from infected substances to the food or bodies of the inmates, the sewer air simply predisposing the system to infection.

A few words now about different methods of sewage disposal are necessary to enable us to treat intelligently the house plumbing connecting with and dependent upon it.

Mr. Roechling** well defines "decomposition" as the process of complete oxidation or mineralization, in which the organic matter in the presence of an ample supply of oxygen is converted into its new compounds of water, carbonic acid, nitrous and nitric acids without the creation of foul smells; and "putrefaction" as the process of incomplete oxidation, in which, in the absence of oxidation, foul smells are produced "which poison the atmosphere."

Decomposition is conducted by the "aerobes," bacteria which can only exist in the presence of oxygen, but putrefaction with its foul and injurious smells is accomplished by hordes of aenaerobes who "finally perish in the ever increasing carbonic acid or in other substances of their own mak-ing.

**"Sewer Gas and Its Influence Upon Health." H. Alfred Roechling. Biggs & Co., London, 1899.

It is stated that the number of bacteria of decomposition which are found in the average dejecta of an adult male exceeds thirty thousand millions, and it is held that, where disease germs exist among these, they succumb after a short struggle for existence with the swarms of the aerobes.

A properly constructed system of sewers is one which delivers all waste matters at the sewer outlet in a fresh condition, that is in a condition in which they might flow through a perfectly smooth and well washed street gutter without attracting attention by their odor. The sewers must be thus constructed and as free from odors as such a street gutter. To accomplish this, all unscoured areas or chambers in the drainage system and foul dead ends of every description in the house or out of it, must be avoided as centers of putrefaction.