107 



from the truth : and he objects to Mr Davy's mode 

 of ascertaining the residual air of the lungs after a 

 forced expiration, from a supposition that the hy- 

 drogen gas which he inspired for that purpose was 

 not, in consequence of its low specific gravity, uni- 

 formly diffused through all the cavities of the lungs : 

 and therefore, that the proportions of the gas dis- 

 charged could furnish no accurate estimate of those 

 which were retained *. But Mr Dalton has shewn, 

 that hydrogen gas and atmospheric air intermix, 

 when the former is kept in a phial above the latter, 

 and communicating only by the small tube of a to- 

 bacco-pipe ; and both in a state of rest f How 

 much more readily then may this be expected to 

 take place, where the gases are exposed to so large 

 a surface, such great agitation, and increased tem- 

 perature, as they must have been in the experiments 

 of Mr Davy. Neither is the small quantity of air, 

 which Mr Davy assigns, so incompatible, as Dr 

 Bostock supposss, with the anatomical structure of 

 the thorax ; for if we call to mind the space which 

 the heart and the lungs occupy, and recollect, that, 

 under a violent exertion, the chest is made to con- 

 tract in every direction, and more especially by the 

 ascent of the diaphragm nearly ,to the fourth or fifth 

 rib, there is no difficulty in imagining the quantity 

 of air in the lungs, in such circumstances, to be near- 

 ly that which Mr Davy's experiments assign. 



* Essay on Respiration, pp. 17. 25. 

 f Manchester Memoirs, vol. i. new series. 



