THE REIGN OF LAW 157 



burning bodies, the other incapable of supporting 

 combustion. He called the first "oxygen." In his 

 Elements of Chemistry Lavoisier gave a clear ex- 

 position of his system of chemistry and of the 

 discoveries of other European chemists. After his 

 studies the atmosphere was no longer regarded as 

 mysterious and chaotic. It was known to consist 

 largely of oxygen and nitrogen, and to contain in 

 addition aqueous vapor, carbonic acid, and ammonia 

 which might be brought to earth by rain. 



Cavendish obtained nitrogen from air by using 

 nitric oxide to remove the oxygen, and found that 

 air consists of about seventy-nine per cent nitrogen 

 and about twenty-one per cent oxygen. He also by 

 use of the electric spark caused the oxygen and ni- 

 trogen of the air to unite to form nitric acid. When 

 the nitrogen was exhausted and the redundant oxygen 

 removed, " only a small bubble of air remained un- 

 absorbed." Similarly Cavendish had found that water 

 results from the combination of oxygen and hydrogen. 

 Watt had likewise held that water is not an element, 

 but a compound of two elementary substances. Thus 

 the great masses, earth, air, fire, water, assumed 

 as simple by many philosophers from the earliest 

 times, were resolving into their constituent parts. At 

 the same time other problems were demanding solu- 

 tion. What are the laws of chemical combination? 

 What is the relation of heat to other forms of energy? 

 To the answering of these questions (as of those from 

 which these grew) the great manufacturing centers 

 contributed, and no city more potently than Man- 

 chester through Dalton and his pupil and follower 

 Joule. 



