FACTORS GOVERNING NUMBER AND KIND 337 



measure of the number of dust particles. An examination of 

 the air of the London streets showed it to contain between 300,000 

 and 500,000 dust particles per cubic centimeter, but there was 

 only one microorganism to every 38,300,000 dust particles. The 

 number of species present will vary with the locality, but probably 

 in the majority of cases it is not great. Fischer states that an 

 examination of the street dust in the city of Freiburg showed it 

 to contain from five to seventeen different species of bacteria in 

 1 gram of dust which contained from 24,000 to 2,000,000 organisms 

 per gram. Graham Smith found at the top of the Clock Tower of 

 the House of Parliament in London only one-third the number 

 of bacteria that were found at ground level. Whipple found 

 1330 bacteria per cubic foot in air at street level, while at the tenth 

 story of the John Hancock Building in Boston the air contained 

 330. 



Factors Governing Number and Kind. The sprinkling of the 

 streets greatly increases the number of bacteria in the dust, but 

 it decreases the number in the air. This is due to the fact that 

 the moist particles are not dislodged and carried into the air as 

 freely as are the dry. 



The air of the .country contains fewer bacteria than does the air 

 of the city. Miquel found as an average 300 organisms per cubic 

 meter of air taken outside the city of Paris and 5445 bacteria per 

 cubic meter of air taken within the city. 



The number of bacteria in air over oceans is low and varies 

 with the nearness to land. Close to shore there are often very 

 many, while at great distances from land the air may be free from 

 bacteria. 



On mountain tops, in deserts, and in other uninhabited regions 

 the air is nearly free from bacteria. The classical illustration of 

 this fact is found in the experiments carried on by Pasteur to 

 refute the doctrine of spontaneous generation. He exposed flasks 

 containing organic infusions in various localities. Of 20 such 

 flasks exposed to the air of Mer de Glace 19 showed no contamina- 

 tion. In similar experiments Tyndall exposed 27 flasks containing 

 beef infusion to the air of the Aletsch Glacier (8000). None 

 showed contamination, whereas 90 per cent, of a similar number 

 opened in a hayloft did. 



The number of organisms in air decreases with the altitude as 

 well as locality. Jean Binot did not find a single microorganism 

 in 100 liters of air taken on the summit of Mount Blanc. The 

 number rapidly increased on descending. 



On the summit 



At the Grand Plateau 6 



At the Grand Malet 8 



At the Pla'ce de 1'aiguille 14 



At the Mer de Glace 23 



At Montanvert 49 



22 



