A HISTORY OF SCIENCE 



the mixture was likewise equal to a similar mixture of 

 nitrous and common air. . . . The next day I was more 

 surprised than ever I had been before with finding 

 that, after the above-mentioned mixture of nitrous air 

 and the air from mercurius calcinatus had stood all 

 night, ... a candle burned in it, even better than in 



common air.'* 



A little later Priestley discovered that "dephlogis- 

 ticated air ... is a principal element in the composition 

 of acids, and may be extracted by means of heat from 

 many substances which contain them. . . . It is likewise 

 produced by the action of light upon green vegetables ; 

 and this seems to be the chief means employed to 

 preserve the purity of the atmosphere." 



This recognition of the important part played by 

 oxygen in the atmosphere led Priestley to make some 

 experiments upon mice and insects, and finally upon 

 himself, by inhalations of the pure gas. "The feeling 

 in my lungs," he said, "was not sensibly different from 

 that of common air, but I fancied that my breathing 

 felt peculiarly light and easy for some time afterwards. 

 Who can tell but that in time this pure air may become 

 a fashionable article in luxury? . . . Perhaps we may 

 from these experiments see that though pure dephlogis- 

 ticated air might be useful as a medicine, it might not 

 be so proper for us in the usual healthy state of the 

 body." 



This suggestion as to the possible usefulness of oxy- 

 gen as a medicine was prophetic. A century later the 

 use of oxygen had become a matter of routine practice 

 with many physicians. Even in Priestley's own time 



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