DISCOVERY BY EXAMINING NEGLECTED SUBSTANCES. 501 



feeble effect of gravity upon the physical forces, and our 

 ignorance of a sufficiently conspicuous instance of such 

 effect, that an experimental connection between the two 

 has not yet been discovered. Similar methods may be 

 employed for the discovery of extreme cases and conspicuous 

 instances, as have already been recommended for that of 

 exceptional ones. 1 



/. By examining common but neglected substances. 

 There is nothing absolutely worthless for the purposes 

 of discovery. Glauber, the chemist, a discoverer of several 

 chemical compounds, said he made it a rule to examine 

 what every other chemist threw away. Oxygen was once a 

 neglected though common substance. Eck de Sulsbach, 

 nearly 300 years before Priestley, heated six pounds of an 

 amalgam of silver and mercury, and converted the latter 

 into a red oxide looking like cinnabar ; and he remarked, 

 ' A spirit is united with the metal ; and what proves it is 

 this, that this artificial cinnabar, submitted to distillation, 

 disengages, that spirit.' The ' spirit ' was oxygen. Whe- 

 ther Priestley knew or not of this experiment we cannot 

 tell ; but in the year 1774 he placed some oxide of mer- 

 cury upon the top of quicksilver, in an inverted glass 

 tube filled with that metal and standing in mercury, and 

 heated the oxide by means of a glass lens and the sun's 

 rays, and obtained a gas. When he first obtained it he 

 did not know what it was, and called it ' nitrous air,' 

 because, like that compound, it rekindled a red-hot splint 

 immersed in it ; and he had to investigate its nature by 

 means of additional experiments before he found what it 

 really was. Similarly, by investigating the common sub- 

 stance black oxide of manganese, Scheele, in the follow- 

 ing year, also discovered oxygen ; he further discovered 



1 See pp. 98 and 199. 



