CHAPTER XIII. 

 EXAMINATION OF WATER, SOIL AND AIR. 



A pure water-supply is of first importance in the con- 

 servation of public health. That impure water is the cause 

 of many diseases is. to-day a well recognized fact. It has 

 ceased to be a theory. Cholera and typhoid iever are 

 striking examples of water-borne diseases. It should not 

 be inferred, however, that these diseases are always con- 

 veyed through drinking-water. The specific germs may, at 

 times, be introduced into the body by means of various 

 articles of food or by direct contact of the mouth with 

 infected articles. Nevertheless, the most common vehicle by 

 which these organisms enter the body is the drinking-water. 



A pure water, therefore, is essential to the prevention 

 of these and other diseases. Until very recent times an 

 opinion as to the purity of a wciter was based wholly upon a 

 chemical examination. ' In some instances, the water may 

 be injurious because of the presence of poisonous metals 

 or other injurious compounds. Thus, the presence of 

 arsenic or lead, or of large amounts of copper or zinc would 

 certainly render the water impure and highly dangerous to 

 health. In such cases the chemical examination will di- 

 rectly reveal the presence of the poisonous metal. As a 

 striking illustration of this, may be mentioned the fact that 

 Vaughan detected relatively large amounts of arsenic and 

 antimony in the river "water of a Western mining region. 

 The water had been used by animals, a large number of 

 which, as a result, died from arsenical poisoning. 



The real danger in water, however, does not lie in the 

 presence of poisonous chemical substances. Intoxications, 



