142 HISTORY OF CHEMISTRY. 



from this, that the latter substance consists of the 

 former, combined with a certain air, which, being 

 thus fixed in the solid body, he called fixed air 

 (carbonic acid gas). He found, too, that magnesia, 

 caustic potash, and caustic soda, would combine 

 with the same air, with similar results. This dis- 

 covery consisted, of course, in a new interpretation 

 of observed changes. Alkalies appeared to be made 

 caustic by contact with quicklime : at first Black 

 imagined that they underwent this change by ac- 

 quiring igneous matter from the quicklime; but 

 when he perceived that the lime gained, not lost, in 

 magnitude as it became mild, he rightly supposed 

 that the alkalies were rendered caustic by impart- 

 ing their air to the lime. This discovery was 

 announced in Black's inaugural dissertation, pro- 

 nounced in 1755, on the occasion of his taking his 

 degree of Doctor in the University of Edinburgh. 



The chemistry of airs was pursued by other 

 experimenters. The Honourable Henry Cavendish, 

 about 1765, invented an apparatus, in which aerial 

 fluids are confined by water, so that they can be 

 managed and examined. This hydro-pneumatic ap- 

 paratus, or, as it is sometimes called, the pneumatic 

 trough, from that time was one of the most indis- 

 pensable parts of the chemist's apparatus. Caven- 

 dish 2 , in 1766, showed the identity of the properties 

 of fixed air derived from various sources; and 

 pointed out the peculiar qualities of inflammable 



2 Phil. Tram. 1706. 



