58 life's beginning on the earth 



From these and thousands of other chemical reactions the 

 conclusion is reached that elements invariably combine in 

 a hard and fast ratio. This experience is summed up, and 

 at the same time explained, by the atomic theory which 

 states : 



1. Every element is made up of invisibly small particles 

 which remain unbroken in any chemical reaction: 

 the atoms. All of the atoms of a particular element 

 have the same weight. 



2. Chemical compounds are formed by the union of atoms 

 of different elements in a simple ratio. When two 

 atoms unite each furnishes its proportion to the total 

 weight. 



Although we have mentioned only two examples to il- 

 lustrate the formation of chemically known compounds, it 

 is easy to imagine how many thousands of other diversified 

 substances have been formed by uniting the atoms of the 

 ninety-four elements. And in all this enormous number of 

 possible combinations the simple atomic relations hold good, 

 with one outstanding exception to which we now turn : the 

 compounds of carbon. 



3. THE COMPOUNDS OF CARBON 



Carbon compounds are of the greatest importance in life 

 processes. When dealing with them we are, first of all, 

 confronted with the striking observation that their number 

 and variety is enormously greater than that of all other 

 compounds. This observation has gradually come to light 

 in the development of the science of chemistry during the 

 past century: the total number of all the compounds which 

 contain the element carbon is more than ten times that of 

 the number of compounds of all the other elements. A 

 survey of the enormously extended chemical literature of 

 our times shows that about 300, 000 carbon compounds 

 have been made so far; but there are fewer than 20,000 that 



