94 NOT ALTOGETHER ABOUT PLANTS 



posed of various combinations of these elements ; that all 

 matter, therefore, is composed either of elements or of 

 compounds of elements. This theory, too, has proved to 

 be a sound one. It is accepted to-day as fact. A com- 

 pound, therefore, is any substance whose molecules are 

 composed of two or more elements. If the molecule of 

 a substance cannot be reduced into two or more elements, 

 then the substance itself is an element. 



As compared with the immense number of compounds 

 which exist, the number of elements which have been dis- 

 covered seems quite small. There are only about eighty. 

 Probably some have not yet been discovered, yet there 

 is good reason to believe that the list will never be very 

 much lengthened. A few of the known elements have 

 been discovered only within the last ten years. Thou- 

 sands upon thousands of compounds have been examined 

 and all but about eighty have been reduced to simpler 

 substances. All but about eighty are composed of mole- 

 cules in which two or more elements are united together. 

 These eighty, then, are the elements out of which all sub- 

 tances, so far as we know, are composed. 



Some elements are quite familiar and common. Coal 

 is one of the forms of a common element. This element, 

 carbon, assumes other forms than coal. Diamonds are a 

 form of carbon. Diamonds and coal are both solids. 

 Thus we note that the same substance, even in the same 

 state of matter, may have very different aspects. This is 

 because the molecules which compose them may be dif- 

 ferently related to each other. By very intense heat and 

 pressure pure coal has actually been transformed into pure 

 diamonds. The expense of the process is, however, very 

 great, and the diamonds produced are very small. 



