RESPIRATION. 



of moiilened empty bladders ; and, indeed, it is the ufual 

 effect produced in the air by every moiilened animal fub- 

 itance. If, therefore, the moiftened bladder be thus ca- 

 pable, by itfelf, of acting on the air, we are entitled to 

 conclude that it exerts the fame aftion when it is filled with 

 blood ; and as, On this fuppofition, the oxvgen gas will 

 unite with the carbon, fumimed directly by the bladder, 

 we have no ground whatever to fuppofe this carbon to come 

 from the blood. Hence, therefore, when black blood 

 is reddened by the air, through the coats of a moiftened 

 bladder, the air yields no oxygen to the blood, nor acquires 

 from it any carbon ; but the carbon of the bladder, by its 

 combination with the oxygen of the air, pafles into a ftate 

 of carbonic acid gas. Further Inquiry, § 581 — 587. 



That this change in the colour of blood is always ac- 

 companied by a correfponding change in the air, may be 

 farther inferred from other experiments of Dr. Prieftley. 

 He found that, when the black craffamentum of blood was 

 covered by ferum or milk, it neverthelefs acquired a florid 

 hua, on being expofed to the air (Obf. on Air, vol. iii. 

 p. 370.) ; and Dr. Wells obferved, that a covering of al- 

 bumen, alfo, did not prevent the action of the air on the 

 blood. (Phil. Tranf. 1797.) Now we know that ferum 

 and albumen convert the oxygen gas of the air into car- 

 bonic acid ; and we found by experiment, that the fame 

 effeft was produced by milk, as probably would be the cafe 

 with moft of the animal fluids. Hence it is evident, that 

 when the blood, in the experiments of Dr. Prieftley, be- 

 came florid, through feveral inches of ferum, the oxygen 

 gas mult have been at once changed by it into carbonic 

 acid, and could never, therefore, in the form of oxygen, be 

 conveyed through this fluid to aft on the blood. 



On the other hand, Dr. Prieftley found a thin ftratum 

 of water to prevent entirely this aftion of air on the blood. 

 (Obf. on Air, vol. iii. p. 370.) M. Cigna found the fame 

 thing to take place, when a pellicle of oil was inter- 

 pofed (ibid.) ; and Dr. Wells afcribes a iimilar effeft to a 

 folution of gum arabic. Thefe fubftances, however, aft 

 little, if at all, in changing the air ; and no change of co- 

 lour, therefore, takes place in the blood. That black 

 blood fhould have the power of attracting the oxygen of the 

 air, through feveral inches of ferum, and vet l.ofe this power 

 when a thin ftratum of water is interpofed, feems foinewhat 

 furprifmg, if the intervening fluid be, in each cafe, con- 

 fidered to be equally paffive ; but proceeding on the faft, 

 that the ferum exerts an action on the air, which the water 

 is incapable of effecting, a new circumitance comes into 

 view, and upon it the colouration of the blood may pro- 

 bably depend. 



If, then, it appear, that the interpofition of fubftances 

 between the blood and the air neceffarily prevents that con- 

 taft, which is effential to the chemical union of oxygen with 

 that fluid ; if it alfo appear, that the colour of the blood if 

 never, in fuch cafes, changed, unlefs fuch fubftances be in- 

 terpofed, as are themfelves capable of afting on the air ; 

 and if, laftly, it be proved, that when the blood exhibits 

 this change of colour, the air fuffers a change, and that its 

 oxygen, inftead of combining with the blood, is really con- 

 tained in the carbonic acid that is formed, we muft con- 

 clude, that, whatever be the mode in which the air contri- 

 butes to change the colour of the blood, it cannot be by 

 imparting to it any portion of its ponderable matter. Con- 

 fequently, although thefe fafts prove that oxygen gas pof- 

 feiles the power of changing the colour of the blood, as well 

 through dead as through living animal membranes, yet they 

 afford no evidence of the combination of oxygen with that 

 fluid, but fhew only the converfion of that gas into carbonic 



acid, precifely in the fame manner as this acid gas is formed, 

 when the blood is reddened in the ordinary procefs of re- 

 fpiration. 



Even when the air and blood are brought into contaft, 

 they only exert a reciprocal aftion on each other, by which 

 carbonic acid is formed, but no oxygen appears to combine 

 with the blood. We have already given various proofs, 

 that, when the blood is changed in colour by the agency of 

 the air, the oxygen gas of the air difappears, and carbonic 

 acid is produced. Thefe fafts are confirmed by the experi- 

 ments of M. Berthollet, who confined recent blood in a 

 veffel of common air, and, at the end of twenty-four hours, 

 the air, on analyfis, afforded nearly -,-i- - of carbonic acid. 

 In two other experiments, iimilar refults were afforded ; 

 and in all thefe experiments, the acid gas produced was 

 exaftly equal to the volume of oxygen that difappeared. 

 (Mem. d'Arcueil, torn. ii. p. 462.) Unlefs, therefore, it 

 be maintained, that the fame oxygen can, at the fame time, 

 exift in two combinations, we muft fuppofe, that, in thefe 

 experiments, no oxygen combines with the blood ; and 

 from whatever caufe, therefore, the red colour of the blood 

 may proceed, we may fafely conclude that it cannot arife 

 from the combination of oxygen. Further Inquiry, 



§ 592—595- 



Mr. Ellis confiders that the diminution in bulk, which 

 the refpired air undergoes, according to the relults of molt 

 inveftigations, may be accounted for by the condenfation 

 which oxygen experiences in uniting with charcoal to form 

 carbonic acid. Crawford eftimated this diminution at -ith, 

 Lavoifier at V-th. But the experiments of Allen and 

 Pepys (Phil. Tranf. 1807), performed with a very perfect 

 apparatus, and therefore apparently deferving confidence, 

 do not fupport this notion of condenfation. When they 

 tranfmitted repeatedly oxygen gas over ignited charcoal, fo 

 as to convert it into carbonic acid, the volume was the fame 

 at the end of the experiment as at the commencement. 

 The refearches of thefe chemilts on refpiration, publilhed 

 fince the appearance of the " Inquiry," making it very pro- 

 bable that there is no lofs of bulk in the air refpired, coin- 

 cides with what they have proved concerning the con!titu- 

 tion of carbonic acid. 



Whether the aaote of the infpired air undergoes any change ? — 

 Mr. Murray has brought together all that is known on this 

 point, and arranged it fo clearly, that we avail ourfelvcs of 

 his labours, without any further remark. 



" It is laftly to be determined, what is the influence of 

 the nitrogen of the atmofphere in refpiration ; and with re- 

 gard to this, different conclufions have been formed. La- 

 voifier, in his early experiments, confidered the nitrogen of 

 the atmofpheric air as fuffering no change in refpiration. 

 Mem. de l'Acad. des Sciences, 1777. Memoires de la 

 Societe de Medccine, 1783. 



Prieftley, in the experiments already referred to, in 

 which he breathed the lame air repeatedly, obferved an ap- 

 parent conlumption of its nitrogen, as well as of its oxy- 

 gen ; but he afterwards inclined to the fuppofition, that the 

 deficiency of nitrogen arofe from the greater proportion of 

 it in the lungs after the procefs than before. 



Mr. Davy inveftigated this, and concluded that nitro- 

 gen is confumed in refpiration ; a quantity difappearing 

 equal to about two-tenths of a cubic inch at each natural 

 refpiration, 13 cubic inches being the quantity of air taken 

 into the lungs. As the number of natural infpirations 

 amounted in a minute to 26 or 27, it followed that, in this 

 time, 5.2 cubic inches of nitrogen are confumed ; a refult 

 which wa3 confirmed by continued reipiration, as well as by 

 the refpiration of animals confined in a portion of air ; 



though, 



