228 
SECOND BOOK. 
WHAT BECOMES OF OUR FOOD (p. 163). 
Process of Digestion. — Digestion is carried on in the mouth, 
the stomach, and the small intestine. 
1. The mouth. The teeth masticate, or crush the food. The saliva 
moistens it, and turns part of the starch (which is insoluble) into sugar 
(which is soluble). From the mouth the food passes by the gullet into — 
2. The stomach. Here the gastric juice acts on the nitrogenous part 
of the food. The digested food passes into the small blood-vessels in the 
wall of the stomach. The rest of the food passes on into — 
3. The small intestine. Here two juices pour in, namely the bile and 
the pancreatic juice . These dissolve the fats, and the latter also completes 
the digestion of the starch and nitrogenous foods. The nourishing part of the 
foods then pass into the blood through the small vessels of the intestines. 
THE AIR, WE BREATHE (p. 166). 
Composition Of Air.- — Air is a mixture of several gases, of which 
the chief are nitrogen and oxygen. Small proportions of carbonic acid 
gas, ammonia, and other gases are also present, and vapour. 
Oxygen is without colour, taste, or smell. It is the most abundant of all thr‘ 
elements in nature. Substances that burn in air burn much more rapidly in oxygen 
Nitrogen is also colourless, and without taste or smell. It will neither burn noi 
cause other substances to burn. By its presence in the air it modifies the action oi 
the oxygen. 
Carbonic acid gas is without colour or smell, but has a slight acid taste. It is a 
heavy gas, and will not support life. 
Carbonic AcdcJ Gas. — The carbonic acid gas in the air is thus 
produced : 
1. .By the breath in^ of all animals. 
The air that is breatheu out contains more carbonic acid and water vapour than 
that which is breathed into the lungs. 
The skin also gives off carbonic acid gas and water. 
2. By combustion. 
3. By the decay of vegetable and animal remains. 
Carbonic acid gas is removed from the air by plants, which in sunlight absorb it 
and give out oxygen. The fall of rain also removes a little of the carbonic acid gas, 
as well as solid impurities. 
WHY THE WIND BLOWS (p. 169). 
Movements of the Air.— The land, after being warmed by the 
sun, gives out warmth to the air that is in contact with it. Heated air 
expands, and thus any given volume of air will not always have the same 
weight, but will become lighter as it grows warm. Being lighter it rises, 
