234 EIGHTEENTH CENTURY. FT. in. 



he passed some of it through water and found that it did not 

 dissolve as ' fixed air ; does ; but what surprised him still 

 more was that a candle put into it burnt with a large 

 vigorous flame, and a piece of red-hot charcoal burst into 

 flame in it and burnt furiously. It was clear then that this 

 could not be either ' fixed air ' or ' inflammable air,' for 

 neither of these would feed a flame. He next put two mice 

 into some of the gas, and he found that they lived much 

 longer than in ordinary air. When he breathed it also into 

 his own chest he felt singularly light and easy for some time 

 afterwards. * Who can tell,' he writes, ' whether this pure 

 air may not at last become a fashionable luxury ? As yet 

 only two mice and myself have had the privilege of breathing 

 it.' 



Here, you see, we have come back again to Mayow's 

 fire-air, so long forgotten, which supports life and flame. 

 Priestley had learnt more about it than Mayow had, for he 

 had collected it separately, had burnt it, and breathed it with- 

 out other air being mixed with it ; and he had moreover 

 shown that it could be driven out of metallic compounds, 

 for mercury is a metal. Yet it is disappointing to learn that, 

 in spite of having gone thus far, Priestley was so imbued 

 with Stahl's theory of ' phlogiston/ that he did not really 

 understand the great discovery he had made, but called his 

 gas ' dephlogisticated air/ or air which had lost that 

 imaginary ' phlogiston ' which was always confusing men's 

 minds. 



There is no doubt that he discovered the gas and showed 

 that it was the chief actor in combustion and respiration, 

 and for this discovery and that of an immense number of 

 other gases, he was elected a member both of the Royal 

 Society and the Academic des Sciences, and his fame was 



