Fig. 35. Samples of disease germs supposed to exist in an unventilated sewer.

I made the trip just as you see presented in the picture. The truck in which we rode was propelled by the water itself, containing in this case, perhaps, one part of sewage to 2,000 parts of water. It had a sliding gate descending from the rear of the car into the sewage against which the stream pressed, the car wheels running on the sidewalks on each side. We found the place quite as sweet and clean, quite as light, and quite as quiet as our famous Boston "Subway" and we were not once "held up" during the entire trip by pathogenic bacilli, either singly or in gangs, as some of the ladies evidently expected to be. I have drawn more "windows" in the sewer than there really were, because they should have been there, and in the future undoubtedly will be. The walls should be lined with white enameled bricks or tiles. Properly ventilated it would be the best place to carry gas, light, water and other pipes and wires.

The other picture, Fig. 35, is a correct drawing of a sewer built in an uncomfortable place described by Bunyan in his "Pilgrim's Progress," and contains a great many germs of an exceedingly ferocious disposition, and is described by Bunyan as the place to which all those plumbers are consigned who have, during their lifetimes, imposed upon a confiding public a larger number of waste pipes (called by Bunyan "wasted" pipes) than the conditions actually require.

By sewer air is meant the air of drainage systems, and consists in a mixture of air with vapor and a number of gases of the decomposition of sewage in varying proportions according to the system of waste disposal employed, together with floating solid matters, and a small number of bacteria.

The most important of these gases of decomposition are carbon dioxid, carbon monoxid, ammonia, sulphuretted hydrogen, carbonate of ammonia, ammonium sulphide, methyl sulphide, and a number of volatile organic compounds, which, though they give sewer-air its peculiar odor, are present in too small quantities for accurate chemical determination. Average sewer air in properly constructed sewerage systems contains a less number of micro-organisms than the external air of cities, and far less than the air of crowded rooms, because these organisms, being "particulate" are retained by the water and damp surfaces of the sewers when they come in contact with them. According to the careful and elaborate experiments on the comparative bacteria of sewer air and sewage itself in the sewers of Laws and Andrewes* made for the London County Council, and of numerous other investigators whose testimony we shall quote, it has been found that the microorganisms in the air are totally unlike those in the liquid below it, whereas they do correspond with the organisms in the air outside of the sewers.

The air of sewers contains less oxygen than exterior air, because some of it combines with carbon in converting the sewage into carbon dioxid.

I shall use the term "sewer gas" to designate the air of cesspools and foul sewers, as distinguished from, the air of modern well-constructed and well ventilated sewers. More correctly speaking it should designate the actual gases of decomposition before it is mixed with pure air at all.

Of the chemical constituents of the air of sewers, more or less diluted with normal air, carbon dioxid† is the gas usually found in the largest volume, and is the invariable product of all organic decomposition. It is the "choke damp" of coal mines, and is fatal when inhaled in a highly concentrated form. Being heavier than air, it tends to fall to the ground and remain in the sewers rather than rise from them into the house, unless a strong current induced by heat or other agency raises it. In well-ventilated sewers this gas is incapable of producing the ordinary phenomena of so-called "sewer-gas" poisoning.

*.T. Parry Laws and F. W. Andrewes, London County Council Reports, No. 216, 1895.

†Carbonic acid of the older chemistry.

The weight of this carbon dioxid is clearly shown by the simple experiment of blowing cigarette smoke into a glass tube and observing the manner in which it settles to the bottom of the tube, the exhaled air being largely composed of this gas.

Carbon monoxid* is present in sewers only in exceedingly minute quantities, and even then it is generally due to. the leakage of illuminating gas into them.

Nitrogen, found in sewer air, is incapable of supporting life when uncombined with oxygen, but it is not known to possess any poisonous properties, and in fact its amount in sewers varies little from that in ordinary air.

Sulphuretted hydrogen is a poison when imbibed in no stronger mixture with air than one part in 250, but its odor is so powerful and disgusting that its presence in very small quantities would be insufferable, and it is not found in well ventilated sewers in quantity as great as in an ordinary laboratory where the gas is generated without injury to the operators. Indeed, as Baylies says, a laboratory would not smell natural without it, and yet chemists constantly breathing it, have not been found to suffer any more from typhoid and gastric fevers, cholera, diarrahea, general debility or any other of the derangements supposed to be due to the inhalation of sewer air, than those who never enter a laboratory.

"Students of analytical chemistry have been made sick by inhaling sulphuretted hydrogen, but not seriously, and yet a house in which the smell of this gas was as strong as it usually is in many laboratories at any hour of the day or night, would be considered untenable."

As for ammonia, it may be inhaled in doses far stronger than it ever occurs in properly constructed and ventilated sewers, not only without injury, but if we may judge from the fondness shown by ladies for the smelling salts bottle, with very positive benefit.