622 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 hot contain microdrganisms, 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 aéroscopes 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 ata 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 germsin 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. Thenum- 
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 
