AIR, SOIL, AND WATER. 149 



If a hundred litres of air pass through the tube in fifteen min- 

 utes the germs should all be arrested in the first sand filter. 



And when the filters are removed and thoroughly mixed with 

 gelatine, each filter for itself, there should be no colonies de- 

 veloped from the second filter, t. e., the one nearest the aspirator. 



Varieties Found in Air. The only pathogenic ones found with 

 any constancy are the staphylo-coccus aureus and citreus ; but 

 any bacterium can be, through accident, lifted into the atmo- 

 sphere, and in certain places-may be always found the bacillus 

 tuberculosis, for example, in rooms where many consumptives 

 are living. 



Non-Pathogenic. The micrococci predominate. Sarcina, 

 yeasts, and moulds constantly contaminate cultures. 



In the ordinary habitations the average number of germs to 

 the litre of air does not exceed five. 



Around water-closets, where one would imagine a great num- 

 ber to exist, owing to the undisturbed condition of the air, but 

 few will be found. 



Examination of Water. The bacteriological examination of 

 water is to-day of as much importance as the chemical analy- 

 sis, and must go hand in hand with it. 



At the start we must say that a water containing thousands 

 of germs to the cubic centimeter is far less dangerous than one 

 containing but 2 germs, if one of these two be a typhoid bacil- 

 lus. It is not the number that proves dangerous ; it is the 

 kind. 



If a natural water contains more than 500 germs to the cubic 

 centimeter, it were well to examine its source. 



Bacteriology performs the greatest service in testing the devices 

 which are intended to render water fit for drinking. 



As a diagnostic aid the examination is of but little use. An 

 epidemic of typhoid fever occurs, the water is suspected, an ex- 

 amination is undertaken ; but the days of incubation and the 

 days passed before the water is analyzed have given the typhoid 

 germs, if any had been present, ample time to disappear, since 

 in water that contains other bacteria they live a very short time 

 only. Again, the water tested one day may be entirely free and 

 the next day contain a great number, and before the typhoid 



