CHEMISTRY OF GASES. 273 



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 quick- 

 lime : at first Black imagined that they underwent this change by 

 acquiring 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 imparting 

 their air to the lime. This discovery was announced in Black's inau- 

 gural dissertation, pronounced 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 

 Honorable 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 apparatus, or as it is sometimes 

 called, the pneumatic trough, from that time was one of the most 

 indispensable parts of the chemist's apparatus. Cavendish, 3 in 1766, 

 showed the identity of the properties of fixed air derived from various 

 sources ; and pointed out the peculiar qualities of inflammable air 

 (afterwards called hydrogen gas), which, being nine times lighter than 

 common air, soon attracted general notice by its employment for rais- 

 ing balloons. The promise of discovery which this subject now 

 offered, attracted the confident and busy mind of Priestley, whose Ex 

 periments and Observations on different kinds of Air appeared in 

 1744-79. In these volumes, he describes an extraordinary number 

 of trials of various kinds ; the results of which were, the discovery 

 of new kinds of air, namely, phlogisticated air (azotic gas), nitrous 

 air (nitrous gas), and dephlogisticated air (oxygen gas). 



But the discovery of new substances, though valuable in supplying 

 chemistry with materials, was not so important as discoveries respect- 

 ing their modes of composition. Among such discoveries, that of 

 Cavendish, published in the Philosophical Transactions for 1784, and 

 disclosing the composition of water by the union of two gases, oxygen 

 and hydrogen, must be considered as holding a most distinguished 

 place. He states, 4 that his " experiments were made principally with 

 a view to find out the cause of the diminution which common air is 

 well known to suffer, by all the various ways in which it is phlogisti- 

 cated." And, after describing various unsuccessful attempts, he find? 



3 Phil. Trans. 1766. Phil. Trans. 178-1, p. 119. 



You II. 18. 



