220 ESSENTIALS OF BACTERIOLOGY 



conveyed through the air by those who are infected by coughing 

 and expectorating. 



Non-pathogenic. The micrococci predominate. Sarcinae, 

 yeasts, and molds constantly contaminate cultures. 



In the ordinary habitations the average number of germs to 

 the liter 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 bacteriologic examination of 

 water is to-day of as much importance as the chemical analysis, 

 and must go hand in hand with it. 



A water containing thousands of germs to the cubic centi- 

 meter is far less dangerous than one containing but two germs, 

 if one of these two be a typhoid bacillus. 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 

 examination 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 but a few days 

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

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

 germ can be proved to be present in that particular water the 

 epidemic may be past. Human sewage contamination is 

 determined by finding the colon bacillus, and if this is found 

 in the course of an epidemic of typhoid the water containing 

 it may well be suspected as being the cause. 



Purity of Waters. The purest water we have is the natural 

 spring-water water that has slowly filtered its way through 

 various layers of gravel and sand and comes finally clear and 

 sparkling from the ground. It is without germs; but let such 



