78 ORIGIN AND NATURE OF LIFE 



and at a lower point still these two gases unite 

 together firmly with great discharge of 

 energy. Lower still the gaseous water con- 

 denses into liquid form. What happens for 

 water happens for thousands of other com- 

 binations of the chemical elements which 

 are known as chemical compounds. Each 

 compound contains a definite number of 

 atoms of each of two or more chemical ele- 

 ments. 



The simplest of the compounds, and those 

 first formed in complexity, are the so-called 

 binary compounds which contain only two 

 elements present in such amounts as corre- 

 spond to some simple atomic relationship. 

 For example, water consists of two atoms 

 of hydrogen and one atom of oxygen, and 

 common salt consists of one atom of a 

 metallic substance called sodium, united to 

 one atom of a gas called chlorine, so that 

 it possesses the chemical name of chloride of 

 sodium. 



In order to understand something of this 

 stage in evolution it becomes necessary to 

 explain two specific or individualized pro- 

 perties which here begin clearly to differentiate 

 the various forms of matter, and are called 

 I chemical affinity and chemical valency. This 

 is the first definite inorganic dawning of that 



