THE HONGKONG GOVERNMENT GAZETTE, 27TH SEPTEMBER, 1890. 965
7. No such accumulation and stagnation can take place, in any properly constructed sewer, and the sewage, even if received, already putrescent, from defective house-drains; passes through such sewers so quickly, that but little true gas can be evolved, in transitu. Indeed the chemical composition of air taken from any ordinary sewer differs inappreciably from that of the air outside. In most cases, the most refined analytical methods would probably fail to establish any difference whatsoever.
8. What does take place is that the sewage, especially putrid sewage, imparts a most disagreeable smell to the air with which it comes in contact. In some cases also, it imparts to it most poisonous qualities. The quantity of odori- ferous matter, in sewage-tainted air, though painfully apparent to the nose; and even capable of producing immediate physiological effects, nausea, and so forth, is so minute as to elude the most delicate analytical methods. Just as a drop of attar-of-roses will scent a whole room; or to use a more suitable parallel the smell of cooking cabbage or onious will infect a whole house or neigbhourhood. Yet the chemist would have, to say the least, great difficulty in detecting the odoriferous principle of roses, cabbages or onions, in the air of the room, house or district. The nose is more sensitive than the balance of the analyst.
9. Nevertheless the fact remains that a small amount of sewage will impart a bad smell to a large quantity of air. The smell is stronger and more offensive, when the sewage is putrescent, than when it is fresh; when concentrated, than when amply diluted. It is, moreover, far from certain that the odoriferous emanation from sewage is the thing that produces disease. Indeed it seems probable that it is not. The disease-producing agent is usually supposed to be a yeast- like germ; having its origin or habitat in the sewage; but so far its existence in the air of sewers has not yet been demonstrated. It is only inferred from the fact that such organisms are found in the bodies of persons suffering from diseases, attributed to sewer-tainted air or water. Air smelling of sewage has been in contact therewith, and may, and frequently does, contain these subtle disease-producing particles, whatever they may be, for undoubtedly it ofter produces disease. Certain it is that sewage-tainted air cannot be breathed for a continuance, without danger to health.
10. Before proceeding to discuss the remedy for sewer smells, some facts may be mentioned as to the motion of air in sewers. It is popularly supposed that so called "sewer gas" has a tendency to ascend; probably because it is assumed to be, like some of the better known gases, lighter than air. Lofty ventilators are erected; and they are sup- posed to emit continuously, an invisible, but offensive and deadly cloud, like smoke from a chimney. But the air in sewers differs inappreciably from the air outside them, and its movements obey; not the arrows inscribed on plans by the draftsman, but the general physical laws of motion. Like everything else in the universe, it remains at rest, until it is acted upon by some unbalanced force, and when thus acted upon, it moves in the direction of the force.
11. If the air in an inclined or vertical pipe becomes warmer; and consequently lighter, than the air outside it, there is an unbalanced pressure at the lower orifice, and the air ascends and escapes at the top. If there be a source of heat within the tube, sufficient to heat and rarify each successive portion of air as it enters, a steady upward current will be the result. In an ordinary chimney, the fire supplies the heat, but even in this case, the upward current is not always as steady as could be wished. Chimneys sometimes smoke.
12. If the air in the tube is colder and denser than the air outside, then it will descend, and escape at the lower opening. If there were something in the tube, a lump of ice for example, that would cool the entering air, then a descending current would be the result. Owing to eddies, produced by buildings and the like, the wind may cause a greater pressure on one opening than on the other, and a movement, from the opening where the pressure is greatest, towards that where it is least. Lastly the passage of a current of water through the tube may induce, by friction, a cur- rent of air in the same direction as that of the water.
13. The causes which produce movements of air in sewers are slight, and not easy to trace. But as a matter of fact, downward currents are as often indeed more often observed than upward. This was the result of a series of experiments, made with self-registering apparatus, by Mr. SANTO CRIMP, on the sewers of Wimbledon. These observations which extended continuously, over a whole year, showed conclusively the preponderance of the downward current. My own observations, here and elsewhere, though neither as exhaustive or conclusive as those of Mr. CRIMP, tend to the same conclusion. One case, which happened in Malta, is instructive. A certain ventilating shaft some 50 feet higb, and very conspicuous, was supposed to pour forth "sewer gas" continuously, so as to infect the neighbourhood. The Committee appointed to enquire into the Cholera Epidemic of 1887, determined to see whether this ventilator (among others) did continuously pour forth sewage-tainted air. A man-hole, near to its connection with sewerage-system, was opened and some burning matter was inserted into the lower part of the veutilating pipe. A strong downward current was observed, so much so that the smoke drove the men out of the sewer. More fuel was added till a roaring fire was established; and the man-hole cover was then put on. The fire was maintained for two hours, but not until the expiration of that time did its heat disturb the condition of thing in the sewers sufficiently to establish an upward draft, and cause smoke to appear at the summit of the ventilator. This experiment does not shew that there never was an upward current. It merely shews that there may be a strong and persistent downward current; and it demonstrates the difficulty, amounting almost to an impossibility, of producing an artificial current through a system of sewers in any desired direction. On this point, another experiment of Mr. SANTO CRIMP may be cited. He closed every known opening to the Wimbledon