550 BACTERIA IN THE AIR. 



with the glass tube containing the granulated -sugar filter by a piece 

 of rubber tubing. Instead of transferring the soluble filter to gela- 

 tin in test tubes, they use a large glass cylinder having a slender 

 stem, in which the sugar is placed (Fig. 193). After the aspiration 

 liquefied gelatin is introduced into the large glass cylinder, which is 

 held in a horizontal position ; the sterilized cotton plug is then re- 

 placed in the mouth of the cylinder, the sugar is pushed into the 

 liquefied gelatin and dissolved, and by rotating the cylinder upon a 

 block of ice the gelatin is spread upon its walls as in an Esmarch roll 

 tube. For convenience in counting the colonies lines are drawn upon 

 the surface of the cylinder, dividing it into squares of uniform di- 

 mensions. 



GENERAL RESULTS OF RESEARCHES MADE. 



As already stated, the presence of bacteria in the atmosphere de- 

 pends upon their being wafted by currents of air from surfaces where 

 they are present in a desiccated condition. That they are not carried 

 away from moist surfaces is shown by the fact that expired air from 

 the human lungs does not contain microorganisms, although the in- 

 spired air may have contained considerable numbers, and there are 

 always a vast number present in the salivary secretions. The moist 

 mucous membrane of the respiratory passages constitutes a germ 

 trap which is much more efficient than the glass slide smeared with 

 glycerin used in some of the aeroscopes heretofore described, for it 

 is a far more extended surface. As a matter of fact, most of the sus- 

 pended particles in inspired air are deposited before the current of 

 air passes through the larynx. 



Air which passes over large bodies of water is also purified of its 

 germs and other suspended particles. The researches of Fischer 

 show that at a considerable distance from the land no germs are 

 found in the atmosphere over the ocean, and that it is only upon ap- 

 proaching land that their presence is manifested by the development 

 of colonies upon properly exposed gelatin plates. 



Uffelmann found, in his researches, that in the open fields the 

 number of living germs in a cubic metre of air averaged two hundred 

 and fifty, on the sea coast the average was one hundred, in the court- 

 yard of the University of Rostock four hundred and fifty. The num- 

 ber was materially reduced after a rainfall and increased when a 

 dry land wind prevailed. 



Frankland found that fewer germs were present in the air in 

 winter than in summer, and that when the earth was covered with 

 snow the number was greatly reduced, as also during a light fall of 

 snow ; the air of towns was found to be more rich in germs than the 



