330 BLACK. 



false theory, of the alkalis and alkaline earths, was 

 owing to the loss of a substance with which they had 

 been combined, and that their reunion with this sub- 

 stance again rendered them mild. But the nature of 

 this substance was likewise ascertained by him, and its 

 detection forms by far the most important part of the 

 discovery, for it laid the foundation of chemical science. 

 He found that it was a permanently elastic fluid, like 

 air in some of its mechanical qualities, those of being 

 transparent or invisible, and incondensable, but differ- 

 ing entirely from the air of our atmosphere in its 

 chemical properties. It was separated from alkaline 

 substances by heat, and by the application of acids, 

 which, having a stronger elective affinity for them, 

 caused it to be precipitated, or to escape in the aeriform 

 state ; it was heavier than common air, and it gave a 

 slight acidulous flavour to water on being absorbed by it; 

 hence the inference that it was an acid itself. A short 

 time afterwards (in 1757) he discovered that this peculiar 

 air is the same with that produced by the fermentation 

 of vegetable substances. This he ascertained by the 

 simple experiment of partially emptying in a brewer's 

 vat, where the fermenting process was going on, the 

 contents of a phial filled with lime-water. On shaking 

 the liquid that remained with the air that had entered, 

 he found it become turbid, from the lime having 

 entered into union with the air, and become chalk. 

 The same day he discovered by an experiment, equally 

 simple and equally decisive, that the air which comes 

 from burning charcoal is of the same kind. He fixed a 

 piece of charcoal in the broad end of a bellows nozzle, 

 unscrewed ; and putting that in the fire, he inserted 



