, 3 4 THE VITAL PROCESSES 



10. Show that digestion is not a simple process like that of dissolv- 

 ing salt in water. 



Elements supplied by the Foods. The following brief study will 

 enable the pupil to identify most of the elements present in the body 

 and which have, therefore, to be supplied by the foods. 



Carbon. Examine pieces of charred wood, coke, or coal, and also 

 the " lead " in lead pencils. Show that the charred wood and the coal 

 will burn. Recall experiment (page 114) showing that carbon in burn- 

 ing forms carbon dioxide. 



Hydrogen. Fill a test tube one third full of strong hydrochloric 

 acid and drop into it several small scraps of zinc. The gas which is 

 evolved is hydrogen. When the hydrogen is coming off rapidly, bring 

 a lighted splinter to the mouth of the tube. The gas should burn. 

 Hold a cold piece of glass over the flame and observe the deposit of 

 moisture. Hydrogen in burning forms water. Extinguish the flame 

 by c&vering the top of the tube with a piece of cardboard. Now let the 

 escaping gas collect in a tumbler inverted over the tube. After hold- 

 ing the tumbler in this position for two or three minutes, remove and, 

 keeping inverted, thrust a lighted splinter into it. (The gas should 

 either burn or explode.) What does this experiment show relative to 

 the weight of hydrogen as compared with that of air? 



Nitrogen. Nitrogen forms about four fifths of the atmosphere, 

 where, like oxygen, it exists in a free state. It may be separated from 

 the oxygen of an inclosed portion of air by causing that gas to unite 

 with phosphorus. Place a piece of phosphorus the size of a pea in a 

 depression in a flat piece of cork. (Handle phosphorus with wet fingers 

 or with forceps.) Place the cork on water and have ready a glass fruit 

 jar holding not more than a quart. Ignite the phosphorus with a hot 

 wire and invert the jar over it, pushing the mouth below the surface of 

 the water. The phosphorus uniting with the oxygen fills the jar with 

 white fumes of phosphoric oxide. These soon dissolve in the water, 

 leaving a clear gas above. This is nitrogen. Place a cardboard under 

 the mouth of the jar and turn it right side up, leaving in the water and 

 keeping the top covered. Light a splinter and, slipping the cover to 

 one side, thrust the flame into the jar of nitrogen, noting the effect. 

 (Flame is extinguished.) Compare nitrogen with oxygen in its relation 

 to combustion. What purpose is served by each in the atmosphere? 



