into a fever. This fever is accompanied by abnormal chemical changes within his tissues and the production of toxic substances, which in turn react upon his tissues diminishing their working power, inducing early fatigue, and upsetting the normal equilibrium of his organs. The result of such a disturbance of his bodily mechanism, if very pronounced, is the production of a pathological condition which is called heat stroke.
But the extreme condition of air at a temperature above the bodily temperature and completely saturated is not necessary for inducing the pathological symptoms. We may witness them under somewhat more moderate conditions in the frequent cases of sunstroke which occur in the streets of our American cities on hot and humid days. The observations of Kubner, one of the foremost German hygienists, indicate that even at 75° F., or more than 23 degrees below bodily temperature and with a humidity of only 80 per cent, of saturation an untrained man can continue comfortable only by refraining from physical work. The performance of work under these conditions would throw a tax upon his powers of adaptation. Even at still lower degrees of temperature and humidity the unfavorable symptoms may begin to appear, indeed the point at which our environing air ceases to be comfortable and begins to force us to make special efforts at accommodation to it is one that is not outside our range of frequent experience.
Many experiments, some of them striking, seem to make it clear that it is to these two features of heat and humidity, the same features which are responsible for sunstroke, and not to others, that all the evil effects of the air of crowded, ill-ventilated rooms are actually due. These experiments have usually consisted in confining and observing men, perhaps several together, in comparatively small experimental chambers. Sometimes these chambers have been little more than bare boxes; sometimes they have been rooms provided with elaborate devices for varying the quantity and qualities of the air. Sometimes the subjects of the experiments have been obliged to breathe over and over again the same air; sometimes the air has been kept under careful control and changed in various ways. The effects of the various conditions have then been observed and recorded. These observations upon human beings have been supplemented by a variety of experiments on animals, and these animal experiments have added greatly to our knowledge of the qualities of the air which human beings ought to breathe.
One of the notable and fruitful investigations was an American one, carried on between 1893 and 1895 by Billings, Mitchell and Bergey with the aid of the Smithsonian Institution in Washington. The Billings of this investigation was the efficient organizer and first librarian of the New York Public Library and the Mitchell was our famous physician-author, Dr. Weir Mitchell. Another helpful American contribution is that of Benedict, whose work with the respiration calorim-
