The accumulation from year to year of similar records brings the evidence closer and closer to the value of positive demonstration, and at any rate it will be wise for the public to accept them as such inasmuch as they constitute the best proof to be obtained until individuals are found public spirited enough to voluntarily surrender themselves in the interest of science to such experimentation as has been made by Dr. Alessi* on animals.

We come now to the other forms of sewer-gas poisoning designated by Hankel as the "mild" and the "fairly severe" forms. But before presenting a few of the recorded cases, it is important as a preparation for properly understanding them to review the recent researches of Dr. Alessi on the effect of sewer-gas in predisposing the system to special disease infection. His experiments were made upon animals of different kinds, and resulted in showing that after exposure to the influence of putrid gases, including sewer-gas, inoculation with the germs of certain diseases killed them, but that these germs failed to kill when the animals had been kept under normal condition.

He took rats, guinea pigs and rabbits, and exposed some of them to sewer and other putrid gases. The rest he kept as a control experiment, under normal conditions, and after a while inoculated all of them with the baccillus of typhoid fever and the bacterium coli communis, and then he carefully observed and recorded the results produced on both sets of animals, including microscopic examinations with cultures of their organs and blood.

In a second set of experiments he studied "whether the chemical substances which are commonly given out in a state of gas from putrid fermentations can also exercise separately a similar influence on the animal organism."

*G. Alessi "On Putrid Gases as Predisposing Causes of Typhoid Fever," Journal of the Sanitary Institute," 1895. Vol. XVI, p. 487.

The experiments were conducted with so much care, thoroughness and precaution that they cannot fail to convince the reader of the correctness of his conclusions, which Dr. Alessi states as follows:

"From my researches, taken together, I think I am authorized to conclude as follows:

"I. The inspiration of putrid gases predisposes the animals (rabbits, guinea pigs, rats) to the pathogenic action of even attenuated typhoid bacilli, and of bacterium coli.

"2. This predisposition is due to the combination of gases given out by putrid fermentations, and not to any one. separately.

"3. It is probable that this experimental predisposition is diminished by prolonged breathing of the said gases.

"These conclusions, then, serve to confirm what some authors had epidemiologically foreseen, and social hygiene had practically and painfully confirmed."

The gases which, taken separately, were found not to predispose the animals to typhoid infection, were retilindol a very strong smelling product of putrefaction of albuminous substances, ammonia, sulphuretted hydrogen, methyl sulphide, carbonic acid, carbonic oxide, and ammonium sulphide. "Not only," says Dr. Alessi, "did the gases taken separately, have no predisposing effect, but even some of them, when mixed, for which reason I may be allowed to suppose that both the exhalations arising from faecal and the exhalations arising from organic matter in putrefaction, are not composed of simple mixtures, but are much more complicated than might be believed. And the predisposing cause might also have its seat in those fetid substances of neutral character, which it is impossible either to understand or determine, whether from their small quantity, the insufficiency of analytical methods, or from the imperfection of those which we have."

Dr. Alessi experimented on 312 animals, of which 179 were exposed to the sewer-gas and 133 were kept in fresh air. Of the exposed animals 143 died when inoculated with the bacteria; whereas of the animals not exposed only three died, all of which were rats.

He found that the animals acquired the predisposition to infection more easily during the first two weeks than after that time, which may explain in a certain degree why individuals who habitually breathe sewer-air become habituated to it and acquire a certain immunity from intestinal and other infections.

Sanitary engineers have frequently raised the question for discussion whether it will be best to seek security by perfecting our sewerage system to the extent of rendering the air within it as innocuous as the outer air, or whether we must, as it were, "bottle up" the sewers under the assumption that sewer-air must always be dangerous under any degree of dilution with pure air, or exposure to sun light, and direct our energies to confining sewer-gases to the sewers themselves, a course which must necessarily tend to vastly increase the expense and danger of house plumbing.

It seems to me that difference of opinion on this subject is unjustifiable in view of the data which scientists have now prepared for us.

All sewers should be thoroughly ventilated and their air rendered entirely innocuous. House drains should aid in ventilating the sewers, the disconnecting or main house trap and all "back venting" should be rigidly prohibited by law, and the whole interior piping system be vastly simplified.

I base my position, in part, on the recent researches and discoveries of a great many distinguished modern investigators who are in accord in concluding that the number of germs in sewer-air. is small and less than in outside air; that the bacteria found in sewer-air are not the same in kind as those found in the sewage itself, but are the same as those found in the air outside the sewers; that disease germs are and demonstrably must be rarely found in sewer-air and can live but a very short time in sewage itself; that disease germs cannot detach themselves from the surface of water at rest, nor from the damp surfaces of sewers; that bacteria in sewer-air, whether reaching the sewers from the external air or from the bursting of bubbles or drying of the sewage, tend to fall by gravity into the water of sewers or to be driven by air currents against moist surfaces, in the sewers and drain pipes, from which they cannot again arise spontaneously that sewers can be and frequently are so well constructed and ventilated that the air within them becomes entirely innocuous; and in part from my own investigations and conclusions. By taking advantage of modern discoveries and progress a simple system of house drainage without the disconnecting trap and back venting can be made perfectly safe whereas the complicated system now generally in vogue cannot be so made.

Thus we find that plumbing has become a science based upon some of the most profound and delicate researches, in both the visible and the invisible world and that the arrangement of our piping is governed by the habits of the minutest living beings known to the microscope.

The following conclusions referred to in our introductory chapter have been practically accepted by all investigators as demonstrated: •

(I) Dust and germs cannot rise from wet surfaces or from water or sewage under normal conditions. Abnormal conditions in sewers producing splashing and bubbling may in practice allow a few germs to escape into the air, but these conditions may be obviated by proper construction and regulation.

(2) A sound water seal forms a reliable barrier against sewer-air and germs.

(3) It is possible that germs may be lifted by air currents in sewers from dried surfaces, but so strong a current appears to be required for this that in practice the number may be considered as negligible from the point of view of the sanitarian.

(4) Disease germs do not live long in sewage in com-' petition with other germs.

(5) Fewer germs of any kind are found in the air of sewers than in the outer air above them.

(6) Disease germs may abound in the outer air, but in sewer-air their number is so small as to be negligible from a sanitary standpoint.

From my own investigations and the above conceded facts I feel justified in drawing the following deductions:

(1) The back venting of traps destroys their seal by evaporation and renders them useless as barriers against sewer-air.

(2) Splashing and bubbling in sewers may be prevented by proper construction and regulation.

(3) The use of disconnecting traps shuts off the only effective method of sewer ventilation and forces foul sewer-air into the streets.

(4) The best way to filter the air of cities and towns oft dust and germs is to ventilate the sewers through the house drains in which these impurities are caught and destroyed.

(5) Permanently reliable pipe jointing is obtainable.

(6) Hence all danger from sewer-air may now be avoided by such ventilation and the use of reliable traps and piping.

It remains now to review, in brief detail, the investigations of some of the highest authorities leading up to these conclusions, and my own experiments and reasoning from which they are deduced.