118 ESSAYS BIOGRAPHICAL AND CHEMICAL 



was Scheele. They noticed that when certain substances 

 were heated, gases or, as they termed them, 'airs' escape. 

 For it had been supposed that all gases, as we now name 

 them, were merely modifications of ordinary air ; just as 

 we sometimes notice a pleasant or a disagreeable smell, 

 and attribute it to the ' goodness ' or ' badness ' of the air, so 

 it was generally thought that gases, such as coal-gas, were 

 a sort of air with an unpleasant odour and the curious 

 property of catching fire. 



About fifteen years before Priestley and Scheele made 

 their great discovery of oxygen, the constituent of air 

 which supports combustion, a Scottish professor, Joseph 

 Black, investigated the particular kind of 'air' which 

 escapes when chalk or limestone is heated. And he made 

 the great discovery that this ' air ' can be reabsorbed by 

 lime the residue left after chalk is heated so that 

 chalk is again formed. 



Moreover, he weighed the chalk before it was heated, 

 he measured the gas, and he weighed the lime left after 

 the gas had been driven off from the chalk. And lastly 

 he weighed the chalk which was re-formed after the lime 

 had absorbed the gas. 



He found that the lime was lighter by just as much 

 as the gas weighed ; and he called this gas ' fixed air/ to 

 emphasise the fact that it could be ' fixed ' or absorbed by 

 lime and similar substances. 



This first opened the way for the investigation of gases ; 

 it was a great discovery perhaps one of the most fertile 

 which has ever been made. It is to be noted that Black 

 was not content with this, however; for he recognised 

 that the fixed air from chalk was of the same nature as 

 steam from water. And just as it was necessary to heat 

 water so as to drive it into steam, so it appeared to him 

 that carbonic acid gas, to give ' fixed air ' a more modern 

 name, was a gas by virtue of the heat or ' caloric ' which it 



