39 



volumes as before, the combining proportions of nitrogen and oxygen. 

 One of these experiments well illustrates the rigorous precision of his 

 chemical ideas. It is difficult to bring the whole of a given quantity 

 of nitrogen into combination by the electric spark ; in the preceding 

 experiments the proportion of that gas which had disappeared with the 

 oxygen had been observed ; but there was a residue ; and while that 

 remained Cavendish was dissatisfied with the evidence : we know ni- 

 trogen, he argued, only either by negative properties — that it does not 

 burn — that it does not support respiration, — or by the relative property 

 of its specific gravity : these properties cannot give an absolute cer- 

 tainty that it is not a mixed gas : its entire combination can alone 

 prove that it is unmixed ; he therefore devised an experiment by which 

 he contrived to convert the whole of the quantity operated upon into 

 nitric acid, and thus he established with respect to it, what he had esta- 

 blished before respecting hydrogen and oxygen, the fact of its sim- 

 plicity as a combining body. What I mean by its simplicity as a com- 

 bining body, will be understood by observing that he continues to use 

 just the same hypothetical language respecting nitrogen as before, and 

 still speaks of it as a compound of nitric add and phlogiston. On his first 

 discovery of the composition of water he had entrenched the old doctrine 

 of phlogiston in the only position which was any longer tenable : he 

 fought the same battle for it as was fought in later days for oxymuriatic 

 acid, and on the same grounds : as I have shown that whenever Caven- 

 dish used the term dephlogisticated air, oxygen may be safely substituted 

 for it, and whenever he uses the term phlogiston, hydrogen* may be 

 substituted for it, I am entitled to show the similarity of the reasoning 

 in these cases more distinctly by explaining it in modern terms : " As 

 dephlogisticated air \_oxygeny\ he argues, "is only water deprived of 

 phlogiston \_hydrogen'], it is plain that adding dephlogisticated air \_oxy- 

 gen~\ to a body is equivalent to depriving it of phlogiston \_hydrogen~\ and 

 adding water to it, and therefore phlogisticated air [nitrogen, supposed a 

 compound of dry nitric acid and hydrogen~\ ought also to be reduced to 

 nitric acid by being made to unite to, or form a chemical combination 

 with dephlogisticated air [oxygen'] ; only the acid formed this way loill 

 be more dilute than if the phlogisticated air was simply deprived of phlo- 

 giston [hydrogen] f-" 



Thus did Cavendish, in deference to received opinion, speak of ni- 

 trogen as a compound, while at the same time he was taking pains to 



* It is worthy of remark that Cavendish was the first chemist who identified 

 phlogiston with hydrogen. Phil. Trans, vol. Ivi. 1766. 

 t Phil. Trans. 1785^ p. 379. 



