BLACK. 337 



in nature, only careful observation was required to 

 find them out. Inflammable air was the next which 

 became the subject of examination, because, though it 

 had long been known, before Black's discovery it had 

 been supposed only to be common air mixed with unc- 

 tuous particles. His discovery at once showed that it 

 was, like fixed air, a separate aeriform fluid, wholly 

 distinct from the air of the atmosphere. The other 

 gases were discovered somewhat later. But it is a very 

 great mistake to suppose that none of these were known 

 to Black, or that he supposed fixed air to be the only 

 gas different from the atmospheric. The nature of 

 hydrogen gas was perfectly known to him, and both 

 its qualities of being inflammable and of being so much 

 lighter than atmospheric air ; for as early as 1766 

 he invented the air balloon, showing a party of his 

 friends the ascent of a bladder filled with inflammable 

 air. Mr. Cavendish only more precisely ascertained 

 its specific gravity, and showed what Black could not 

 have been ignorant of, that it is the same, from what- 

 ever substance it is obtained. 



But great as was the discovery of fixed air, and 

 important as were its consequences, the world was 

 indebted to its illustrious author for another scarcely 

 less remarkable, both from being so unexpected, and 

 from producing such lasting effects upon physical 

 science. About the year 1763 he meditated closely upon 

 the fact, that on the melting of ice more heat seems 

 to disappear than the thermometer indicates, and also 

 that on the condensation of steam an unexpected pro- 

 portion of heat becomes perceptible. An observation 

 of Fahrenheit, on the cooling of water below the tem- 



z 



