156 ELEMENTARY SCIENCE 



particle into which any substance may be divided, and yet 

 remain the same substance. You know that in air, oxygen, 

 nitrogen, and carbon dioxide occur. Now the new fact 

 to note is that air is composed of separate molecules of 

 oxygen, of nitrogen, and of carbon dioxide (CO 2 ), which 

 are equally distributed in accordance with the law of 

 diffusion that you have already considered. In other 

 words, if all molecules were so big that we could see 

 them, and tell them apart one from another, we should 

 see that water is composed of molecules which are all 

 alike. But when we examine air, we find it to be chiefly 

 composed of two distinct kinds of molecules, oxygen 

 molecules and nitrogen molecules, and these would be 

 symmetrically arranged so that you would find one oxygen 

 molecule associated with every four nitrogen molecules. 

 Then, about once in every twenty- five hundred molecules 

 (not counting those of water vapor) you would find one 

 of carbon dioxide. 



Thus we see that a compound is a substance all of whose 

 molecules are of the same kind, while a mixture is a substance 

 whose molecules are of more than one kind. 



Elements. It is the work of chemists to analyze 

 substances, and thereby to discover of what they are 

 composed. In this process molecules are decomposed 

 in order that their composition may be discovered. Now 

 one of the most important and significant facts that all 

 this work of the chemists has revealed is that there are 

 only about eighty substances in the world which may not 

 be reduced into simpler substances. They have analyzed 

 thousands upon thousands of different substances, decom- 

 posing them as far as possible, and the result of all this 



