A LESSON IN CHEMISTRY 9 



Chlorine (Cl) is a heavy, yellow-green, poisonous gas, with a 

 disagreeable, suffocating odor. It is never found free, but is 

 widely distributed in nature in compounds, the most familiar 

 of which is common salt (NaCl). Chlorine apparently takes 

 no part in the life processes of plants, but is common in the 

 compounds used by them. 



Carbon (C) is a black solid, familiar to us in charcoal and 

 mineral coal. Graphite, or black lead, used in pencils, is 

 another form of this element. The diamond, the hardest sub- 

 stance known, is a crystalline form of carbon. Peat and muck 

 are impure forms. Carbon has never been liquefied, but it 

 may be turned to a gas at very high temperatures ; many of 

 its compounds are gases or liquids at ordinary temperatures. 

 When carbon unites with oxygen, heat and light result. In 

 slow combustion, such as that in our bodies, no light is pro- 

 duced, but the amount of heat that finally results is the 

 same as if the substances burned more rapidly. The carbon 

 so abundant in plants is not derived from the soil, but from 

 the small quantity of carbon dioxide in the air. Carbon is 

 believed to occur in more different compounds than any 

 other element. 



Phosphorus (P) is a pale yellow, poisonous substance about 

 as soft as wax and has a disagreeable odor. It does not occur 

 native, but in such combinations as phosphates of lime and 

 aluminum it is present in large quantities in many rocks. 

 It burns with a yellow-white light when exposed to the air, 

 and must be kept under water in the laboratory. Phosphorus 

 is a necessary constituent of the nucleus of plant and animal 

 cells and is also found in considerable abundance in seeds. 



Sulphur (S) is a yellow substance occurring native or com- 

 bined with various elements, as sulphates and sulphides. It 

 melts readily, and burns with a bluish flame and suffocating 

 odor, producing sulphur dioxide (SO 2 ). Sulphur is an essen- 

 tial element in all plant and animal bodies. 



