CHEMISTRY OF GASES. 143 



air, (afterwards called hydrogen gas,) which, being 

 nine times lighter than common air, soon attracted 

 general notice by its employment for raising bal- 

 loons. The promise of discovery which this subject 

 now offered, attracted the confident and busy mind 

 of Priestley, whose Experiments and Observations 

 on different kinds of Air appeared in 1744-79. In 

 these volumes, he describes an extraordinary num- 

 ber 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 respecting their 

 modes of composition. Among such discoveries, 

 that of Cavendish, published in the Philosophical 

 Transactions for 1784, and disclosing the composi- 

 tion of water by the union of two gases, oxygen and 

 hydrogen, must be considered as holding a most 

 distinguished place. He states 3 , that "his experi- 

 ments 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 phlogisticated." And, after describing 

 various unsuccessful attempts, he finds that when 

 inflammable air is used in this phlogistication, (or 

 burning,) the diminution of the common air is 

 accompanied by the formation of a dew in the 

 apparatus 4 . And thus he infers'' that "almost all 



3 Phil. Trans. 1784, p. 119. 4 Ib. p 128. s Ib. p. 129. 



