148 



<jn the circulation of the blood, 



ment of respired air introduced into the air-vesicles, and not retunied by 

 the alternate expiration, ia necessarily conveyed into the blood-vessels, 

 believing that much of this may remain unascertained, in consequence of 

 an increased, but not sensibly increased, expansion of the chest. He admits 

 that carbonic vapour is thrown forth in the quantity usually alleged with 

 every act of expiration ; but he offers evidence to prove that it is the car- 

 bone only that is discharged from the animal system, in connexion with 

 the exhaling vapour ; contending that the carbone thus existing is sepa- 

 rated from the vapour by its union with the whole of the oxygene intro- 

 duced by the previous act of inspiration by which alone it is converted 

 into carbonic acid gas : for he found the same decomposition of atmos- 

 pheric air produced by introducing a small bladder, moistened, and filled 

 with any substance, or perfectly empty, and introduced into an inverted 

 glass containing a certain portion of atmospheric air standing upon quick- 

 silver. He denies, therefore, that the air-vessels are in any degree porous 

 to gases of any kind, excepting caloric ; and, consequently, denies that 

 the blood is converted from a deep modena hue into a bright scarlet by 

 its union with oxygene ; behoving, or seeming to believe, that this regult 

 is entirely produced by the action of the caloric separated in the air-vesi- 

 cles upon the union of the carbone of the vapour exhaled from their sur- 

 faces, with the oxygene introduced by inspiration. So that, according to 

 this theory, respiration is nothing more than an introduction of caloric 

 into the system, and the conversion of a portion of oxygene (the whole 

 received by the act of inspiration) into an equal bulk of carbonic acid by 

 the carbone exhaled from the living organized body. Air, therefore, ex- 

 amined after respiration, is found to differ from the same air before it is 

 breathed in having lost a portion of oxygene, gained an equal volume of 

 carbonic acid, and in being loaded with pure watery vapour, the vapour 

 thrown off from the lungs ; and he has offered an additional proof that 

 the oxygene of the carbonic acid ia that introduced in the act of inspira- 

 tion, by showing, as in the case of breathing hydrogene gas, that no car- 

 bonic acid is returned, and apparently none produced. 



In opposition to the hypothesis of Dr. Priestley, he seems to show, and 

 plausibly to establish, that all terrestrial plants, whether growing in absolute 

 darkness, in the shade, or exposed to the direct rays of the-sun, are con- 

 stantly removing a quantity of oxygene from the atmosphere, atid substi- 

 tuting an exactly equal volume of carbonic acid ; that they produce this 

 change by emitting' from their leaves, flowers, fruits, stems, and roots, and 

 by a process like animal exhalation, carbonaceous matter, which combines 

 with the oxygene of the surrounding air ; and that such a function is essen- 

 tially necessary to their vital existence. In doing this, however, the car- 

 bonaceous matter is given forth more freely from the green parts than from 

 any other, especially when exposed to the direct rays of the sun, by means 

 of its affinity for the calorific rays ; in consequence of which the oxygene 

 of tlie carbone is set at liberty, and escapes from the cellular texture of 

 the green parts through the external pores ; an action, however, which is 

 not necessary to hfe, for a plant does not die when this has ceased, while 

 it is equally found to occur in a dead as in a living plant. It was probably 

 this occasional escape of oxygene that induced Priestley to regard it as 

 an invariable and constant process, affording a compensation for the ani- 

 mal carbone thrown into the air, and thus taking from and giving to the 

 •inimal world what seemed to be mutually demanded. 



ATr. Was also affirms that all the various coioiirs of vegetables depend 



