56o POPULAR SCIENCE MONTHLY. 



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 even- 

 ing materially to alter results. I was even more surprised after ex- 

 posure of plates for two hours in the library, reception hall and game 

 room, to find the most contaminated plate to contain only nine colo- 

 nies. 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 scrupu- 

 lously 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 fol- 

 lowing 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 p.m. 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 



