of soil-pipes, which matter they are constantly decomposing into gases and soluble products readily washed away. They are also present in large quantities in sewage as it flows in the sewers.
There are a few forms of bacteria which we have good reason to believe are the causes of certain diseases called specific. Each of these specific diseases has a definite course, and is due to the entrance into the body of particles of living matter derived, directly or indirectly, from the body of a person affected with the same disease.
We now know the particular kinds of bacteria which cause several of these diseases, and can identify them with considerable certainty. Those of most interest in connection with house-drainage are those which are supposed to cause suppuration, septicæmia, puerperal fever, erysipelas, intestinal irritation and diarrhœa, typhoid fever, and sore throats and diphtheria.
These diseases are less frequent and less fatal in sewered than in unsewered cities, and in the central sewered portion of a city than in the unsewered suburbs. Systematic house-to-house inspections in cities have shown that over one half of the houses have more or less defective and foul fixtures and leaky soil-pipe joints, so that if specific germs are often present there should be much more sickness than there is. As a matter of fact there is no evidence that scarlet fever, measles, small-pox, or whooping-cough has ever been transmitted by sewer air. There is reason to think that in a few and exceptional cases diphtheria and typhoid have been caused by inhaling sewer or soil-pipe air; but the danger of incurring these diseases in this way is small as compared with the other and usual sources of origin, although it is probable that the ordinary non-specific sore throats which sewer air tends to produce form a specially favorable site for the development of the specific microbe of diphtheria, and that in this way foul air is a predisposing cause of this disease. Schools are much more dangerous than sewers as regards the propagation of diphtheria.
The typhoid-fever bacillus is said to have been found in the air of a sewer from an institution in which there was an epidemic of typhoid, and there is a theoretical possibility that the disease might thus be produced in a house by conveyance of its germs through sewer and soil-pipe air; but such conveyance must be extremely rare. It should be distinctly understood that neither the most perfect system of house-drainage nor total absence of house-drainage will protect the inmates of the house to any considerable extent from diphtheria or from typhoid.
The most dangerous micro-organisms which are commonly found in sewer and soil-pipe air are those which produce suppuration, erysipelas, or septic poisoning when they gain access to the