12 BLACK. 



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 na- 

 ture 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 impor- 

 tant as were its consequences, the world was indebted 

 to its illustrious author for another scarcely less re- 

 markable, 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 propor- 

 tion of heat becomes perceptible. An observation of 

 Fahrenheit, on the cooling of water below the tem- 

 perature of ice until it is disturbed, when it gives out 

 heat and freezes at once, appears also to have attracted 

 his careful consideration. He contrived a set of simple 

 but decisive experiments to investigate the cause of 

 these appearances, and was led to the discovery of 'latent 

 heat, or the absorption of heat upon bodies passing from 

 the solid to the fluid state, and from the fluid to the aeri- 

 form, the heat having no effect on surrounding bodies, 

 and being therefore insensible to the hand or to the 

 thermometer, and only by its absorption maintaining the 



