A HARD-WORKING DIET. 9 
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 
: Sets i live on ele- 
Nitrogen as foods, it is only inthe form of compounds ments as 
: such, but 
they come under our notice. We do not,and perhaps only in the 
‘ form of 
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- 
x pounds are 
pounds of two elements pass into plant structures parts of plants 
5 first bef 
first and form more complicated compounds, and we We can use 
them. 
make use of these compounds direct from plants, 
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 
are more 
familiar than 
elements are. 
‘some of three and some of all four—are perhaps more 
familiar to everybody than are the elements them- 
selves, though they are familiar under other names. 
Water is an instance. It is a compound of Waterisa 
Hydrogen and Oxygen, and the chemist calls it ee 
gen-Oxide. 
It results 
_ contains nothing but Hydrogen and Oxygen. But SO al 
the water of our rivers, wells, and springs contains Hydrogen. 
Hydrogen-Oxide—that is, if it is absolutely pure and 
small quantities of other things besides the Hydrogen 
and Oxygen of which it is essentially composed, small 
quantities of matter dzssolved in it, very often lime, 
which makes the water hard. We also loosely, under 
the term “water,” often include small quantities of 
iB) 
