DUST AND ITS DANGERS. 2$ 



The analysis of out-of-doors air shows, as 

 might be expected, a great deal of variation 

 in the number of living germs present in a 

 given volume. Ten litres, which is about 600 

 cubic inches (that is a volume equal to a cube 

 of about 8 inches square), is the volume of 

 air usually taken as a sample for purposes of 

 analysis. 



Carnelly found in still out-of-doors air, in 

 the town of Dundee, in Scotland, as the result 

 of 14 analyses, an average of less than 10 

 bacteria in 10 litres of air, while in another 

 place there were over 1 70 in the same volume. 



Tucker found the air in Boston, from a 

 secluded place, but in the immediate vicinity 

 of its traffic, during the mild but rather windy 

 weather in November, December, and January, 

 with no snow on the ground, to contain on the 

 average of 56 analyses, less than 20 bacteria to 

 10 litres. In an open court at the Hygienic 

 Institute in Berlin, Petri found, as a rule, 

 equally small numbers. 



The average of 13 analyses, made in March 

 and April, 1 890, of the air from the yard of the 

 College of Physicians and Surgeons, in New 



