A HARD-WORKING DIET. 339 



Most people know something about other com- 

 pounds of Nitrogen. Nitric acid, which is a compound 

 of Nitrogen and Oxygen, is an example, and so is 

 ammonia, commonly called hartshorn, which is a 

 compound of Nitrogen and Hydrogen. 



In considering Carbon, Hydrogen, Oxygen and We do not 



live on ele- 



Nitrogen as foods, it is only in the form of compounds ments as 

 they come under our notice. We do not, and perhaps only' in the 

 cannot, live on them as elements. We eat plants compounds 

 (roots, fruits and leaves) and we eat "beasts, birds 



and fishes " that have fed on plants. Simple com- The com- 

 pounds are 

 pounds of two elements pass into plant structures par ts of plants 



first and form more complicated compounds, and we w r e s ca ^ ^ 

 make use of these compounds direct from plants, them- 

 or after they have formed fresh compounds as parts 

 of fish, flesh or fowl. These compounds are very 

 various in their composition, and are various in their 

 uses to us. These uses will be spoken of later on, 

 after more has been said of the compounds themselves. 



Compounds of these elements some of two together, Compounds 

 some of three and some of all four are perhaps more familiaTthan 

 familiar to everybody than are the elements them- elements are - 

 selves, though they are familiar under other names. 



Water is an instance. It is a compound of Water is a 

 Hydrogen and Oxygen, and the chemist calls it caSedHydro- 

 Hydrogen-Oxide that is, if it is absolutely pure and 

 contains nothing but Hydrogen and Oxygen. But 

 the water of our rivers, wells, and springs contains Hydrogen, 

 small quantities of other things besides the Hydrogen 

 and Oxygen of which it is essentially composed, small 

 quantities of matter dissolved in it, very often lime, 

 which makes the water hard. We also loosely, under 

 the term "water," often include small quantities of 



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