EXAMINATION OF AIR, SOIL, AND WATER 235 



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 

 number to exist, but few will be found, owing to the undis- 

 turbed condition of the air. 



Sewer air seldom, if ever, contains bacteria, and neither 

 typhoid fever, malaria, nor diphtheria has ever been traced to 

 the escape of so-called sewer-gas. 



Examination of Water. The bacteriologic examination 

 of water is today 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, and 

 consider it with suspicion. 



As a means of diagnosis the examination is of but little use. 

 An epidemic of typhoid fever occurs, the water is suspected, 

 an examination is undertaken; but the period 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 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 free from bac- 

 teria, but let such a water stand walled up in cisterns or 



