110 MANUAL OF PHYSIOLOGY. 



tasteless, inodorous and colorless when pure. Soft water, such as 

 rain-water, is pure, but not so agreeable to taste as spring water, 

 and is very liable to contamination in its passage over roofs pre- 

 vious to collection. Standing water should be avoided for drink- 

 ing, owing to the probability of its containing organic matter. 



Salts. Great varieties of salts are taken into the system, of 

 which chloride of sodium forms the largest proportion. These 

 have, no doubt, very important functions to perform, in entering 

 into combination with the various tissues, and also probably in 

 aiding the chemical changes of parts of which they do not form 

 a normal constituent. They help to render certain substances 

 soluble, and stimulate the cells of certain glands to more active 

 secretion, e. g., the kidney excretes more urea when there is an 

 abundant supply of common salt in the food. 



