anthrax and suppuration. The mere fact that such germs have at times been found, however, is of little significance in the matter of possible aerial infection. They never occur in any considerable numbers, and considerable numbers of germs are usually necessary to produce a disease. It is known that many bacteria on being cast out into the air from an infected source lose their virulence in the process of drying and soon die. Evidence that disease germs pass through the air from room to room of a house or from a hospital to its immediate surroundings always breaks down when examined critically. It is indeed not rare now to treat cases of different infectious diseases within the same hospital ward. The one place of possible danger is in the immediate vicinity of a person suffering from a disease affecting the air passages, the mouth, throat or lungs, such as a "cold," or tuberculosis. Such a person may give out the characteristic microbes for a distance of a few feet from his body, not in quiet expiration, for simple expired air is sterile, but attached to droplets that may be expelled in coughing, sneezing or forcible speaking. In this manner infection may, and at times probably does, occur, the evidence being perhaps strongest in the case of tuberculosis. But apart from this source there appears to be little danger of contracting an infectious disease from germs that float to us through the medium of the air—aerial infection in the most of those diseases with which we are familiar is, in the authoritative words of Chapin, "under ordinary conditions of home and hospital a negligible factor." Avoid all forms of physical contact with disease germs or germ-laden articles; keep hands and dishes clean; beware of infected food and water; if you can detect him shun the bacteria-carrier, he who unwittingly carries within his body the germs without the disease and may deposit them where subsequent physical contact is possible; but do not be tormented any longer by the unnecessary specter of germladen air.
I might add a few words concerning sewer gas. Sewer gas consists simply of air containing volatile substances which are given off by the decomposing organic matters that occur in sewage. There is nothing mysterious about the components of sewer gas except in the minds of those who stand in dire dread of it. It may contain carbon dioxide, the ill-smelling hydrogen sulphide and ammonium sulphide, marsh gas, ammonia and certain other gaseous substances—all of these in variable, and, with the possible exception of carbon dioxide, usually small amounts. There is no excessively poisonous gas among them. Bacteria exist abundantly in sewage, but these appear to be given off to the air only when the liquid sewage is mechanically splashed and then only in very small numbers and usually not to great distances. Winslow has made a most careful experimental study of this subject, and has come to
