86 ESSAYS BIOGRAPHICAL AND CHEMICAL 



Cullen, his predecessor, in which the boiling-point of 

 liquids had been found to be lowered by reduction of 

 pressure; he rightly ascribes this to the freer escape of 

 the vapour, and to the absorption of heat by the vapour, 

 and the consequent cooling of the liquid from which it is 

 escaping. 



These conceptions of Black's were utilised by his friend 

 James Watt in' his work on condensers, and, as every one 

 knows, effected a revolution in the structure of steam- 

 engines, and as a consequence in the whole of our indus- 

 trial and social life ; and further, they were developed by 

 many men of science, until in the hands of the masters 

 Joule, Clerk-Maxwell, Rankine, James Thomson, and 

 Kelvin, on the physical side, and of Willard Gibbs, the 

 American, on the chemical side they form the very 

 groundwork of the sister sciences, physics and chemistry. 



Black's great chemical discovery that a gas exists which 

 is clearly not a modification of atmospheric air, seeing it 

 can be ' fixed ' by alkalies and alkaline earths, led the way 

 to ' pneumatic chemistry,' as it was called, and was followed 

 by the discovery of oxygen by Priestley, of nitrogen by 

 Rutherford, of hydrogen by Cavendish and Watt, and of 

 the more recent discoveries of argon and its congeners, all 

 of them constituents of the atmosphere. In fact, the gases 

 of the atmosphere have been discovered entirely by Scots- 

 men and Englishmen. 1 



And Black's proof, that the change of a complex com- 

 pound to simpler compounds, and the building up of a 

 complex compound from simpler ones, can be followed 

 successfully by the use of the balance, has had for its 

 consequence the whole development of chemistry. It is 

 only in the most recent years, since Becquerel observed 

 the effect of uranium ores and salts in discharging an 



1 In justice to the Swede Scheele, it should be said that his discovery 

 of oxygen was contemporaneous with Priestley's. 



