14 BIOLOGY FOR BEGINNERS 



both cases an oxide of sulphur (SO 2 ) with the familiar, suffocating 

 odor which we wrongly associate with sulphur itself. 



Its importance in biology is due to the fact that it is a part of 

 the living substance of all organic things though in smaller amounts 

 than any of the preceding elements. 



Mustard, onions, and eggs will blacken silver dishes. This is 

 due to the sulphur compounds which they contain; but sulphur, 

 in smaller quantities, is found hi all plants and animals. 



PHOSPHORUS 



Phosphorus (P) is a light yellow, waxy, solid element. Like 

 sulphur, it dissolves in several other liquids, but not in water. 



It also resembles sulphur in that it unites readily with oxygen. 

 In fact it unites with oxygen more readily than does sulphur, for, 

 if exposed to air, it will take fire and burn fiercely, forming an 

 oxide of phosphorus. It has to be kept covered with water to 

 prevent it from burning and is a dangerous and poisonous element. 



It seems strange that such a substance should be a necessary 

 ingredient of our bodies and, in fact, of all living things. To be 

 sure it is present in small amount but is absolutely essential, 

 being especially abundant in bone and nerve tissue. 



You have probably heard plant fertilizers called " phosphates." 

 This is because they contain phosphorus compounds. 



IRON 



Iron is another element. We are familiar with it as a heavy, 

 solid metal; and we know it unites slowly with oxygen forming 

 iron oxide (rust). This is about the last thing we would think to 

 be of use in the bodies of plants or animals. However, iron is 

 absolutely necessary in the green coloring matter of plants and in 

 the red blood of animals. Later we will learn the remarkable 

 services which its compounds perform in these substances. 



