ix] Doctrines of Respiration. 243 



" phlogiston to the common air or dephlogisticated air, and have 

 " imbibed it and have become saturated with it when exposed 

 "to phlogisticated, nitrous, inflammable, or fixed air." 



It will be observed that Priestley's idea of respiration as 

 being simply the phlogistication of dephlogisticated air left no 

 room for any other product of respiration. Black, we have seen, 

 had clearly shewn that his fixed air was a product of respira- 

 tion, was a constituent of expired air. Priestley (and this shews 

 how far he was from laying hold of the real truth about respira- 

 tion) had to explain away in some manner or other Black's fixed 

 air. He attempts to shew that it does not come from the lungs. 



" It now being pretty clearly determined that common air 

 "is made to deposit the fixed air which entered into the 

 "constitution of it by means of phlogiston in all cases of 

 " diminished air, it will follow that in the precipitation of lime 

 " by breathing into lime water the fixed air, which incorporates 

 " with lime, comes not from the lungs, but from the common 

 "air, decomposed by the phlogiston exhaled from them, and 

 " discharged, after having been taken in with the aliment, and 

 " having performed its function in the animal system." 



Priestley's story is a striking example of the influence of a 

 dominant theory. He was, as we have said, steeped in the 

 phlogiston theory ; he clung to it to the end of his life, though 

 to others it seemed before that to have received its death-blow. 

 From what I have said it is clear that he had formed in his 

 mind an image of the respiratory process which, so far as 

 oxygen is concerned, we with our present knowledge may call 

 wonderfully exact, save that it was, in a sense, completely 

 upside down, an image of the truth, but an inverted image. 

 Where we say ' took,' he said ' give,' and vice versd, and this so 

 persistently throughout the whole business that anyone who at- 

 tempts, as I have just done, to describe respiration in Priestley's 

 terms, will find that he has to be very careful at each step lest 

 he represent him as saying exactly the opposite of what he did 

 say. It is so difficult for us, as it was so easy for him, to think 

 of oxidation as a ' giving up,' and not as a ' taking in.' 



Meanwhile another mind of quite a different mould was 

 laying hold of the truth in its proper, erect position. Priestley 



1G— 2 



