AIR AND WATER COMPARED 157 



work indicates that all substances whatsoever are composed 

 of one or more of these eighty simplest substances. Just 

 as all words are made up of various combinations of a 

 few letters, so all substances are made up of various com- 

 binations of a few elements. So you can see that the 

 translation of this word, analysis, "loose back," gives a 

 good idea of the real nature of the process. It is a process 

 of "loosing" molecules "back" to the elements of which 

 they are composed. The opposite of analysis is synthesis, 

 which means the "putting together" (by chemical combina- 

 tion) of simpler substances into more complex ones. Thus 

 you may have heard of synthetic rubber, which means rubber 

 manufactured by chemical processes out of simpler sub- 

 stances so that the product is practically equivalent to 

 the natural rubber obtained from plants. 



Note that new word, elements. It is important. All 

 substances, then, are elements or compounds of elements. 

 What are these eighty elements or simplest substances? 

 With some of them you are already familiar. Hydrogen, 

 oxygen, and nitrogen are elements. Others with which 

 you are familiar are carbon, copper, gold, iron, lead, mercury, 

 phosphorus, platinum, silver, sulphur, tin, and zinc. Evi- 

 dently, then, elements may exist as liquids, solids, or gases. 

 Which is the element named above which exists in liquid 

 form? 



Atoms. Now to go back again to our water molecule. 

 We have been told that it is composed of two parts of hydro- 

 gen and one of oxygen. These "parts" of which mole- 

 cules are composed are called atoms. Thus the molecule 

 of carbon dioxide is composed of one atom of carbon and 

 two of oxygen. 



