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 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.
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