n. CHEMICAL COMPOSITION OF LIVING 

 MATTER. 



Chemical elements. — The dissection of a plant or 

 animal shows it to be made up of many different kinds 

 of materials, some soft, some hard, some elastic. Dis- 

 section, however, can tell us nothing of the composition 

 of these separate masses of material, and to find out what 

 bone, muscle, and fat are made of, we must go to the 

 chemist. As the result of methods which chemists have 

 learned to apply to the problem, they tell us that muscle, 

 fat, bone, wood, and in fact all the other substances of the 

 universe are built up of simple substances which are called 

 elements. By examining all the different kinds of matter, 

 both living and lifeless, to be found in the universe, it has 

 been found possible to reduce the number of these simple 

 substances or elements to some seventy-six, and just as 

 all the words in the language may be built up of the 

 twenty-six letters of the alphabet, so all matter is built 

 up by varying combinations of these seventy-six elements. 

 Some of these elements are very rare, while others are 

 exceedingly abundant. Some are never found in the 

 pure state but always combined with other elements, 

 while others are found both in a free state and in combin- 

 ation. Of the seventy-six known elements, twelve ^ are 

 always found in living matter and six of the remaining 

 sixty-four arc sometimes present. 



• The twelve that are always present in living matter are phosphorus, 

 sulphur, carbon, oxygen, hydrogen, nitrogen, chlorine, potassium, 

 sodium, calcium, magnesium, and iron. 



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