HUMAN PHYSIOLOGY: INTRODUCTION 293 



toward the bottom. After a few seconds gas will come off. The de- 

 livery tube may then be placed under water beneath the opening of 

 the jar, and soon the bubbles of oxygen coming off will displace the 

 water in the jar. 



Caution. — After collecting a jar of oxygen (or several jars) lift the 

 end of the delivery tube out of the water before removing the lamp, 

 otherwise the water will rush back into the delivery tube and crack it. 



Insert the live coal of a splinter into the jar. It will burst into flame. 



Fig. ISS. Apparatus for collecting oxygen. (After Jenkins and Kellogg.) 



Or heat the end of a piece of picture wire and insert the red-hot wire into 

 the oxygen. It will burn with a bright flame, thus showing again that 

 oxygen "supports combustion." 



Other experiments with oxygen may be found in books on elementary 

 chemistry. 



Properties of Carbon. — Carbon is the chief solid element 

 in wood, muscle, fat, sugar, starch, etc., in fact in every 

 substance that is or has been living. For this reason, sub- 

 stances containing carbon are called organic substances. It 

 is found in coal, showing that coal was once a living sub- 

 stance. A special branch of chemistry, called "Organic 

 Chemistry," is devoted to the study of the carbon compounds. 



The black substance, or charcoal, left after the splinter 

 was burned is almost pure carbon. It is without taste or 

 odor. When carbon is cold it has little affinity for other 

 elements. When it is heated, however, it takes up oxygen, 

 or becomes oxidized, as we have seen in our oxygen experi- 



