PRIESTLEY 61 



In 1774 Priestley discovered ammonia gas too, which 

 he gave the name of alkaline air ; and proved that mixed 

 gases do not separate on standing, but remain uniformly 

 distributed throughout the mass (diffusion of gases). 



By passing electric sparks through ammonia gas, he 

 observed that an inflammable gas was formed ; and that 

 phlogisticated air (the mephitic air of Rutherford and 

 the nitrogen of Chaptal and later chemists) was evolved 

 on heating ammonia with the calx of lead (lead oxide, 

 PbO). Although a brilliant experimentalist, " with rare 

 quickness and perceptive powers," Priestley had not the 

 genius of the philosopher ; " he passed too rapidly from 

 subject to subject even to notice the great truths which 

 lay under the surface " ; he left for others to interpret 

 the results of his discoveries. The phlogistic doctrine was 

 always in the way of this remarkable man ; he never could 

 free himself of its encumbrances, its wordy phraseology, 

 and its false theory. He was blinded by Stahl's doctrine 

 even to the end of his days. He only "looked at the 

 results of his experiments through the fogs of his 

 prejudices " ; but his discoveries proved in the hands of 

 Lavoisier the deathblow to phlogistic ideas. The conflict 

 with error was ended and truth prevailed. His work 

 represents the death of an old science and the birth of 

 a new one. Such was the life of Joseph Priestley, and 

 " le monde comme il va ! '' 



