THE ALPHABET OF ALL LIVING THINGS 13 



Uses. Hydrogen is important to the biologist because it unites 

 readily with oxygen and forms water. It also combines with both 

 oxygen and carbon (another element) and forms a whole series of 

 compounds called fats, sugars, and starches. It is an essential 

 ingredient in all organic tissue. 



CARBON 



Carbon (C) is an element with which we are more familiar; coal, 

 charcoal, and wood are common forms. Lead-pencils do not 

 really contain lead at all but another form of carbon called 

 graphite. Strangest of all, the diamond is carbon, too, though 

 not a common form. 



Properties. Carbon is (except in the diamond) a black solid, 

 not soluble in any thing. At ordinary temperature it is very 

 inactive. When heated, however, it unites readily with oxygen, 

 (that is, it burns) and forms an oxide which is called carbon dioxide 

 a compound very necessary to plants, as we shall see later. 



Uses. Carbon's importance to biology is due to the fact that it 

 is a part of all organic substances, combining with hydrogen, 

 nitrogen, and oxygen and other elements to form all plant and 

 animal tissues and many of their foods. 



We know that if any plant or animal substance is partly burned 

 a black solid is produced. This, in every case, is carbon. We also 

 know that if the burning is continued the carbon will disappear. 

 This means that it becomes oxidized into carbon dioxide, which is 

 an invisible gas. 



Plants alone have the power to obtain their carbon from the 

 carbon dioxide of the air. Animals depend entirely on plant 

 foods for the carbon compounds which are necessary for their life. 



SULPHUR 



Sulphur (S) is a yellow solid element, which (like carbon) will 

 not dissolve in water, but can be dissolved in other chemicals. 



Sulphur itself has no odor, but it readily unites with oxygen, 

 even at low temperatures. It also burns readily, producing in 



