RESPIRATION, AND ANIMALIZATION. 143 



ing- hydrogen gas, that no carbonic 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 constantly 

 removing a quantity of oxygen from the atmosphere, and substituting 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 oxy- 

 gen of the surrounding air; and that such a function is essentially necessary 

 to their vital existence. In doing this, however, the carbonaceous 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 oxygen of the carbon 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 life, 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 oxygen that 

 induced Priestley to regard it as an invariable and constant process, affording 

 a compensation for the animal carbon thrown into the air, and thus taking 

 from and giving to the animal world what seemed to be mutually demanded. 



Mr. Ellis also affirms that all the various colours of vegetables depend on 

 the varied proportion of alkaline and acid matter mixed with the juices of the 

 coloured parts of plants : that green and yellow, for example, are always pro- 

 duced by an excess of alkali in the colourable juices of the leaf or flower ; 

 and all the shades of red, by a predominance of acid; while -a neutral mix- 

 ture produces a white. And hencr there is most green in the summer sea- 

 son, when the oxygen is parted with most freely, as drawn away by the rays 

 of light; while in autumn, when there is less separation, the other colours 

 of yellow and red are most frequent. 



Mr. Ellis has also quoted a variety of experiments on different kinds of 

 fishes, muscles, marine testacea, snails, leeches, zoophytes, and tadpoles, in 

 which it was found that the water wherein these animals had been placed 

 had lost a part of its oxygen, and received an addition of carbonic acid, while 

 its nitrogen had remained unaffected.* 



This hypothesis, however, requires confirmation, and is at present open to 

 many objections. If caloric can permeate animal membranes, as Mr. Ellis 

 admits it to do, and unite by chemical affinity with the blood in the blood- 

 vessels, so also may oxygen in certain cases of combination. Mr. Porrett 

 has shown that the Voltaic fluid, when operating upon water, is capable of 

 carrying even water itself through a piece of bladder, and of raising it into a 

 heap against the force of gravitation ; and hence other affinities may not only 

 introduce the oxygen of the respired air, or a part of it, into the blood of the 

 blood-vessels in the lungs, through the tissue of the air-cells, but at the same 

 time carry off the superabundant carbon in the form of carbonic acid, instead 

 of its being thrown out in that of carbonic vapour. Nor have we any proof 

 that carbon will dissolve in water, and produce such vapour; and hence such 

 an idea is gratuitous.! 



Of the general operation, however, there is no doubt, whatever be the 

 manner in which it is performed : and by such operation the new blood 

 becomes assimilated to the nature of the system it has to nourish ; and the 

 old or exhausted blood both relieved from a material that may be said to suf- 

 focate it, and reinspirited for fresh action. In this state of perfection, pro- 

 duced from the matter of food introduced into the stomach, and elaborated 

 by the gases of the atmosphere, received chiefly by the act of respiration, but 

 perhaps partly also by the absorbing pores of the skin, the blood on its ana- 

 lysis is found to consist of the following nine parts, independently of its aerial 



* Inquiry into the Changes induced on Atmospheric Air by the Germination of Seeds, &c. 8vo. 1807. 

 As also, Farther Inquiries into the Changes, &c. 8vo. 1811. 



Study of Med. edit. ii. vol. i. p. 474. Thomson's Annals of Philos. No. xliii. p. 75 7fl 



