CHEMISTRY OF THE EARTH 81 



Each one of the chemical atoms possesses a 

 different weight, which is utilized as an 

 indicator of the amount of matter in it. Until 

 quite recently the absolute masses of the 

 atoms were unknown to us, it is one of the 

 many triumphs of the study of radio-activity 

 that we now know them, but for ordinary 

 chemical operations it is only necessary to 

 know their relative weights and these relative 

 weights are what are known as the atomic 

 weights of the elements. 



When two or more elements unite to form a 

 chemical compound, the masses taking part 

 in the reaction are always so related that 

 they may be expressed as simple multiples 

 of the atomic weights. This shows that 

 always a definite number of atoms enter into 

 the formation of each compound, and it is for 

 this reason that under similar conditions of 

 environment, all the characters of a chemical 

 compound are absolutely definite and in- 

 variable. Thus two atoms of hydrogen al- 

 ways combine with one atom of oxygen to 

 form what is termed a molecule of water, and 

 never in any other ratio or proportion is 

 water formed. Thus one molecule of water is 

 always precisely similar to another molecule, 

 and, hence, in the mass, or bulk, if water be 

 quite unmixed with anything else, one sample 



