Page:Popular Science Monthly Volume 64.djvu/564

560 On the evening after my first day's work, the daughter of the household gave a party to about fifty young people of Luray, and on making my first examination of the plates, I was so astounded at the results that I immediately prepared more plates, thinking that enough dust and dirt would have been left in the house on the previous evening materially to alter results. I was even more surprised after exposure of plates for two hours in the library, reception hall and game room, to find the most contaminated plate to contain only nine colonies. This was the plate exposed on the mantle shelf immediately above a large open fireplace in the reception hall, between the hours of eleven and one o'clock noon, when the household of about six people was moving about as usual on a winter morning.

Plates were set for two hours in the caverns on the second day at the same places as on the first day. Again two colonies developed on the plate exposed near the Crystal Spring, while the other plates were negative.

For the sake of comparison, and to learn whether all houses in that vicinity contained unusually pure air, I exposed a plate in the house of a well-to-do farmer within a mile of Limair. The house was scrupulously neat and clean. The plate was exposed for one hour on a mantle-shelf back of a heating stove in the sitting room, where five or six people were passing in and out. In other words, the conditions were about the same as those under which nine colonies developed after a two-hour exposure at Limair. After 24 hours of incubation 143 colonies were visible on this one-hour plate.

In a physician's office at Luray 92 colonies were implanted in one hour. I expected to catch more bacteria in a physician's office than in a farm-house, and the difference may possibly be explained by the location of a large, open fireplace opposite the door entering from the street, thus affording a means of constant ventilation, with repeated additions of fresh air from the outside.

Outdoor exposures were made at Limair on a clear morning following a day of rain and freezing temperature. The temperature was 38 degrees and a mild wind was blowing. Four plates were exposed in the pine woods at some distance from the house, the exposures lasting from one to two hours. Three plates each showed two colonies, while the fourth had four colonies; of three unopened control plates one showed one colony. To compare the air in the city, I exposed a plate for one hour on the stone wall surrounding my back yard. The plate was exposed at 5 on a clear, bright day, the temperature being below freezing, and the wind blowing about twelve miles an hour. The previous two or three days had been clear and dry. After 36 hours' incubation at 85° F. 450 colonies had developed on the plate. A plate placed on the seat beside me in a Madison Avenue car, during