PRIESTLEY 145 



among the best in England, the few who really knew him; 

 among these were Boulton, Watt (who almost suffered the 

 same fate), and other eminent persons in industry, science, 

 and art in and near Birmingham, which together formed the 

 Lunar Society (also known as the Lunatic Society), since it 

 always met at full moon. In this society there was much 

 discussion concerning the contemporary progress of science. 

 Priestley was in many respects very similar to Boyle; a 

 relentless seeker and defender of the truth, satisfied alone by 

 the joy in new discoveries and observations, with a youthful 

 enthusiasm for all that he undertook, and deeply religious. 



His own studies of gases began in connection with Black's 

 work on carbonic acid, which he obtained from a brewery. 

 He was the first to introduce the collection of gases over 

 mercury instead of water, which enabled him to recognise 

 and study many gases as peculiar kinds of matter; for these 

 gases, though they must have appeared on occasion before, 

 could not be properly collected and observed. Using our 

 present terms these were nitrous oxide, nitric oxide, carbon 

 monoxide, ammonia, sulphurous acid, gaseous hydro- 

 chloric acid, silicon fluoride. He must therefore be called 

 the discoverer of all these. Oxygen and sulphuretted 

 hydrogen had already been discovered by Scheele, but this 

 was no doubt unknown to Priestley, who published the dis- 

 covery earlier (1774), in his work on the different kinds of air. 



Priestley was also the discoverer of the fact that plants 

 consume carbon dioxide and exhale oxygen, whereby the air 

 always remains constant in its composition and suitable for 

 the breathing of animals; he also showed that this phenom- 

 enon only takes place under the action of daylight. He also 

 investigated the strength of a sound produced by a bell in 

 different gases, and found it to be very feeble in hydrogen, 

 and distinctly louder in carbon dioxide than in air. 



Henry Cavendish belonged to one of the oldest and richest 

 Ls 



