INORGANIC ELEMENTS. 15 



Ex. To about 50 cc. of water in a clean beaker add 2 cc. of the above 

 solution. If the water is quite free from nitrites the reagent imparts no 

 color to it. One hundredth of a milligram of nitrogen as nitrite in the 

 water gives a faint pink color at the end of five minutes ; with large quan- 

 tities the color may become deep rose red. 



Ex. A nitrate test may be illustrated in this manner: To the residue 

 obtained by evaporating 50 cc. of an ordinary river or lake water to dry- 

 ness in a porcelain dish add I cc. of phenolsulphonic acid. Rub the acid 

 over the bottom of the dish, and add a few drops of dilute sulphuric acid. 

 Warm the dish a few minutes and add 25 cc. of water. This should show 

 now a faint yellow color. By supersaturating with ammonia the color be- 

 comes deeper. In this experiment picric acid is at first formed if a nitrate 

 is present and the addition of ammonia yields ammonium picrate, the color 

 of which is more marked. 



For the interpretation of all these tests works on sanitary analysis must 

 be consulted. 



Physiological Importance of Water. This is suggested 

 by the large proportion in which it is present in the animal 

 body, as shown above. It serves primarily as the general 

 solvent for all the solid foodstuffs taken into the system and 

 assists in the removal of the solid waste products or excreta. 

 To accomplish these ends it must be drunk in sufficient quan- 

 tity. It is a well recognized fact that most people in the United 

 States drink too little water, from which various ills result. 

 Important chemical changes within the body are dependent on 

 the so-called hydrolytic action of water. These appear mainly 

 in the phenomena of digestion, in which starches, sugars, pro- 

 tein bodies and fats are altered before absorption, and will be 

 discussed in detail in sections to follow. 



It must be remembered further that water plays a very 

 important part in the removal of heat from the body. For 

 each gram of water evaporated as perspiration or in the 

 breath nearly 600 units of heat are absorbed and in this way 

 over 20 per cent of the heat expenditure may be accounted for. 



The average amounts of water found in the important tissues 

 is shown in the following table: 



Per Cent. Per Cent. 



Dentine 10 Elastic tissue 50 



Fatty tissues 20 Liver 70 



Bones 50 Skin 72 



