ON THE CIRCULATION OF THE BLOOD, 



in almost all ages ; and though we have not yet attained to any thing 

 like demonstration, or even universally acceded to any common theory, 

 the experiments of modern times have established a variety of very im- 

 portant facts which may ultimately lead to such a theory, and clear away 

 the difficulties by which we are still encumbered. 



These facts I shall proceed to examine into in language as familiar as 

 I can employ : I must nevertheless presume upon a general acquaintance 

 with the elementary principles and nomenclature of modern chemistry, 

 since a summary survey of zoonomy is not designed to enter into a detail 

 of its mere alphabet or rudiments, but to apply and harmonize detached 

 facts that relate to it, and to condense the materials that have been col- 

 lected by others into a narrow but regular compass. 



The chief substance which has been ascertained to be introduced from 

 the atmosphere into the air-vesicles of the lungs during the act of respira- 

 tion, and from these into the blood, is oxygene, of which the atmosphere, 

 when pure, consists of about twenty-eight parts in a hundred, the remain- 

 ing seventy-two being nitrogene. 



That this gaseous fluid enters into the lungs is rendered highly proba- 

 ble from a multiplicity of experiments, which concur in proving that a 

 larger portion of oxygene is received by every act of inspiration than is 

 returned by every correspondent act of expiration ; and that it passes from 

 the air-vesicles of the lungs into the blood we have also reason to believe 

 from the change of colour which immediately takes place in the latter, and 

 from other experiments made out of the body, as Well as in the body, 

 which abundantly ascertain that oxygene has a power of producing this 

 change, and of converting the deep purple of the blood into a bright 

 scarlet. 



It is also supposed very generally, that a considerable portion of caloric 

 or the matter of heat, in its elementary form, is communicated to the 

 blood at the same time and in conjunction with the oxygene ; but as this 

 substance has- hitherto proved imponderable to every scheme that has been 

 devised to ascertain its weight, this continues at present a point avowedly 

 undetermined. That an increase of sensible heat at all times accompa- 

 nies an increase of respiration is admitted by every one ; but since caloric 

 may be obtained by other means, if obtainable at all, and since a denial 

 of its existence as a distinct substance has of late years been as strenu- 

 ously urged as it was in former times by the Peripatetic school, and upon 

 experiments inaccessible to those philosophers, we are at present in a state 

 of darkness upon this subject, from which I am much afraid we are not 

 likely to be extricated very soon. 



I have already observed that nitrogene, or azote as it is also called, is 

 the other gaseous fluid that constitutes the respirable air of the atmos- 

 phere. And from a variety of well-conducted experiments by Mr., now 

 Sir Humphry, Davy, it appears also that a certain quantity of this gas is 

 imbibed by the lungs in the same manner they imbibe oxygene, and that 

 like oxygene it is also communicated from the lungs to the blood while 

 circulating through its substance ; for, in the experiments adverted to, he 

 found that, as in the case of the oxygene, a smaller quantity was always 

 returned by every successive act of expiration, than had been inhaled by 

 every previous act of inspiration.* 



The only gas that seems to have been thrown out from the lungs in^ 



♦ Priestley had before shown that nitrogene is absorbed. See Phil. Trans, 1790, p. 106. 



