144 



ed the formation of carbonic acid in respiration by a 

 method somewhat similar : he found, that on breath- 

 ing through an infusion of litmus, the same change 

 to a red colour was produced in it, as when it was 

 exposed to the action of fixed air ; and when, by 

 adding a few drops of the water of potassa, the blue 

 colour was restored to the infusion, it could again 

 be made to dissappear by supersaturating it with 

 the acid expired from the lungs *. 



117. The particular substance which constituted 

 the wholesome part of atmospheric air, was not, how- 

 ever, known to Dr Black at the time his experi- 

 ments were made : and long before the compound 

 nature of the atmosphere was ascertained, it had 

 been supposed by many philosophers, that, to use 

 the language of Bishop Berkeley, " there was no 

 such thing as a pure simple element of air. There 

 is," he adds, " some one quality or ingredient in 

 the air on which life more immediately and prin- 

 cipally depends. What that is, though men are 

 not agreed, yet it is agreed it must be the same 

 thing that supports the vital and the common 

 fiame ; it being found, that when air, by often 

 breathing in it, is become unfit for the one, it 

 \\ill no longer serve for the other. This quality of 

 the air is necessary both to vegetables and animals, 

 whether terrestrial or aquatic ; neither beasts, insects, 

 birds, nor fishes, being able to subsist without air : 

 and when air is deprived of this ingredient, it be- 

 cometh unfit to maintain either life or flame, even 



* Priestley on Air, vol. v. p. 383. 



