222 RESPIRATION. 



as well as water and other liquids, are freely permeable to the 

 different gases. They have also discovered that gases pass 

 through with different rapidity : carbonic acid, for instance, 

 very quickly ; nitrogen, very slowly : whence the different state 

 of the bladder just mentioned, accordingly as carbonic acid is 

 introduced into the vessel in common air, or common air intro- 

 duced into the vessel in carbonic acid. The appearance of car- 

 bonic acid outside a bladder tied over a vessel of venous blood 

 or water impregnated with carbonic acid, and the disappearance 

 at the same time of a portion of the oxygen outside the bladder 

 tied over venous blood, is no less than what occurs to the 

 blood of the lungs in respiration, and the blood in both cases 

 becomes florid. 



The lungs thus seem to serve the purpose, in this respect, of 

 merely exposing an immense surface of blood to the air. Blood 

 could not be so near the air on the exterior of the body without 

 constant injury of the innumerable delicate vessels, nor could 

 the vascular surface be preserved in a moist state, which is neces- 

 sary to the permeability of those vessels. Besides which, suc- 

 cession of air to each point could not be secured. The Creator 

 has therefore wonderfully provided an immense surface within, of 

 the very finest texture, secure from external injury and supplied 

 with constant moisture, and continually exposed not only to the 

 external air, but to successive draughts of it. 



The changes of the blood in respiration are therefore purely 

 chemical, and just the same as occur to venous blood out of the 

 body, in contact with air or separated from it by merely a 

 moistened bladder, and are detailed at page 149. Oxygen unites 

 with the blood ; carbonic acid proportionately escapes. The 

 blood, thus liberated from the cause of its blackness, re-acquires 

 the florid hue occasioned by its salts, but which are not naturally 

 in sufficient quantity to brighten it when much carbonic acid is 

 present. 



Dr. Crawford observed that less carbonic acid was evolved in 

 proportion to the height of the temperature <i ; Dr. Jurine, that 

 more was evolved when the circulation was quickened, during 

 the hot stage of fever, digestion, or exercise, and less in the 

 cold stage'; and his results were confirmed by Lavoisier and 



s On Animal Heat, p. 387. 



T Encycloptdie Methodique, t. i. p. 494. Dr. Prout also observed this effect of 

 exercise before fatigue occurred. 



