ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 239 



especially great on the surface near dwellings, and rapidly decreases 

 with depth, so that at 1 meter down there are few. Ninety per cent of 

 these soil microbes are bacteria, chiefly in the form of spores. It is 

 easy to understand how these may be carried into the air, especially in 

 dry weather, as dust by wind and by evaporative forces. 



It has been calculated that in a town like London or Manchester, a 

 man breathes in during ten hours 37,500,000 spores and germs. 



In Berlin an investigator found 3 colonies of bacteria and 10 molds 

 in 25 liters from an open square, and 37 colonies of bacteria and 33 

 molds from a schoolroom just vacated. Professor Tyndall exposed for 

 a short time 27 flasks containing an infusion of turnip, etc., to air on a 

 ledge of rock above the Aletsch glacier in Switzerland, an altitude over 

 8,000 feet, and then carried them to a kitchen stove with a temperature 

 of 50° to 90° F. In the same way 23 flasks were exposed to the air of 

 a hayloft near the same altitude and placed with the others in the stove, 

 due precautions being taken in all cases to prevent the kitchen air from 

 contaminating the flasks. Of the 27 flasks opened in free air not one 

 showed a sign of organic life; of the 23 opened in the hayloft, 21 were 

 invaded. Many other experiments in London and elsewhere convinced 

 him that the air of an ordinary room siwarnis with germs of life, and 

 that if infusions of flesh, fish, or vegetable be exposed even for a short 

 time to the dusty air they become turbid and putrid within a few days. 

 Exposed for months to air "optically pure," that is, deprived of dust, 

 they remain clear and sweet for months, in fact, do not putrefy at all. 

 Some of the germs or spores in the air have a very remarkable resisting 

 power and will germinate after several hours' boiling; others are killed 

 in five minutes. The spores of bacillus subtilis, which is common in 

 hay or in the air of haylofts, is not killed by prolonged boiling. But 

 bacilli themselves, which are soft and unprotected, are killed by boiling 

 water within a few minutes. The small size of the germs and bacilli 

 may be to some degree realized when we note that in Tyndall's estima- 

 tion the number in a single drop of turbid infusion is probably 500,000,000 

 "many times multiplied." The evaporation of such a drop would then 

 conceivably permit the launch into the atmosphere of more than one 

 thousand million organisms. The natural processes of decay in most 

 places on the surface of the earth must be incessantly nourishing 

 immense numbers of microbes in very great variety, and wherever dry- 

 ing or heating takes place quantities of colonies of all sorts which can 

 flourish in daylight must be raised into the air and widely disseminated. 



Percy Frankland counted the number of microbes falling on a square 

 foot in one minute in several situations, with the following result: 



Roof of Science Schools, Kensington, March 851 



Roof of Science Schools, Kensington, when the wind was stronger 1, 302 



Roof of Science Schools, Kensington , after rain 60-66 



Roof of Science Schools, Kensington, during thick fog 26-32 



Burlington House, during Conversazione 318 



Burlington House, on following morning 109 



