Page:Popular Science Monthly Volume 64.djvu/566

562 a twenty-five minute trip from Broadway to Lafayette Avenue, showed over 1,600 colonies after a thirty-six-hour incubation at 85° F. Plates exposed for one hour in the surgical and gynecologic operating rooms at the Johns Hopkins Hospital showed, respectively, 65 and 58 colonies after incubating 48 hours at 100 degrees. These comparative studies are suggestive, and the studies in the caverns and sanitarium demonstrate that with optical purity or freedom from atmospheric dust, we have air that is practically free from bacteria.

But in spite of the bacteriologic purity of the air in Limair Sanitarium, I am sure many will protest against breathing the polluted, moldy emanations from a source never penetrated by the rays of the sun. I must confess this was my first impression, and the same prejudice has been expressed by many friends with whom I have conversed. But what are the facts, and what is the condition of the caverns' air? In the first place the air is not stagnant. In any part of the caverns the guide's candle, if placed on the floor or on the ledge of a wall, shows by the deflected flame a very decided current of air. Owing to the differences in temperature, there is a constant interchange of air between the caverns and the outside world. This circulation takes place through many natural filters distributed over the hillside in the form of crevices in the rock, which have become filled by porous soil. Both air and water are cleansed in passing through these earth filters. If there be any open fissures for the admission of unfiltered air, its organic particles would soon be deposited on the damp caverns' walls. The action of water passing over and causing the slow dissolution of such a vast surface of limestone can not but be