been confined to a particular part of the house, especially exposed to the effluvia from badly trapped drains, where there could be no doubt as to the source of the infection. The sewer air, laden with the specific poison, may be inappreciable to the senses, but its hurtful effects make themselves felt none the less, and, as recent events have shown, may sometimes exhibit themselves in the most exalted stations of life. Nay, more, it would seem that persons of the upper and middle ranks in towns are more liable to be attacked by typhoid fever than the poor classes; the reason being that the houses of the former are more generally connected with sewers, and either from structure or situation are of higher elevation, so that the light sewer gases, obeying natural laws, are more apt to accumulate in the drains of such houses, and failing efficient trapping and ventilation of the drains, to effect an entrance into the houses themselves. There is good ground for the belief that cholera, diphtheria, scarlet fever, as well as many other diseases, are occasionally spread by means of the air of sewers and cesspools; but whether these diseases originate spontaneously in this way, or whether the sewer gas only serves as a carrier of the disease-germs, is a question, as in the case of typhoid fever, as yet unsettled. _Trades and Manufactures._—Let us next consider some trades and manufactures which have an injurious influence upon persons engaged in them, and to a certain extent upon the community at large. The injurious effects are owing to solid particles and offensive gases which are given out into the air. The result of inhaling air more or less charged with solid particles may be easily explained. When the latter reach the entrance to the windpipe they at once set up irritation in the delicate lining membrane, and nature tries to repel the intruders by the involuntary cough which results. Should this fit of coughing fail in doing so, a quantity of glairy fluid is poured out from small glands in the windpipe, and this fluid enveloping the solid particles tends to prevent them from doing further mischief. Should they, however find their way lower down into the air passages, nature has provided a very beautiful mechanism for their expulsion. The entire lining membrane of these passages is covered with innumerable minute hairs, or “cilia” as they are called, which, by constantly waving in an upward direction toward the mouth, tend to carry the solid particles, and the glairy secretion which they have provoked, away from the lungs, and so out of harm’s way. This wonderful provision of nature is sufficient for the purpose, provided the strain be not too prolonged; but when the supply of irritating particles is constant, or nearly so, the nerves and muscles involved in this mechanism become exhausted and cease to perform this process of expulsion. The irritating particles are now no longer removed from the delicate membrane of the air