RESPIRATION, AND ANIMALIZATION. 141 



mere alphabet or rudiments, but to apply and harmonize detached facts that 

 relate to it, and to condense the materials that have been collected 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 respiration, 

 and from these into the blood, is oxygen, of which the atmosphere, when pure, 

 consists of about twenty-eight parts in a hundred, the remaining seventy-two 

 being nitrogen. 



That this gaseous fluid enters into the lungs is rendered highly probable from 

 a multiplicity of experiments, which concur in proving that a larger portion 

 of oxygen 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 

 oxygen 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 oxygen ; but as this substance has 

 hitherto proved imponderable to every scheme that has been devised to ascer- 

 tain its weight, this continues at present a point avowedly undetermined. That 

 an increase of sensible heat at all times accompanies an increase of respi- 

 ration 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 strenuously 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 nitrogen, or azote, as it is also called, is the 

 other gaseous fluid that constitutes the respirable air of the atmosphere. 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 oxygen, and that, like oxygen, 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 oxygen, 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 the 

 course of these experiments is carbonic acid ; a very minute proportion of 

 which appears also to be almost always contained in the atmospheric air, 

 though altogether a foreign material, probably eliminated from the decompo- 

 sition of animal and vegetable bodies, that is perpetually taking place, and 

 certainly unnecessary to healthful respiration. 



The general result of these experiments was as follows : the natural in- 

 spirations were about twenty-six or twenty-seven in a minute; thirteen cubic 

 inches of air were in every instance taken in, and about twelve and three- 

 quarters thrown out by the expiration that succeeded. 



The atmospheric or inspired air contained in the thirteen cubic inches, 

 nine &nd a half of nitrogen, three and four-tenths of oxygen, and one-tenth of 

 an inch of carbonic acid. The twelve inches and three-quarters of returned 

 air contained nine and three-tenths of nitrogen, two and two-tenths of oxy- 

 gen, and one and two-tenths of carbonic acid. 



This inhalation, however, varies in persons of different-sized chests from 

 26 to 32 cubic inches, at a temperature of 55 ; but these by the heat of the 

 !"angs, and saturated with moisture, become forty or forty-one cubic inches. 



Taking, therefore, 40 cubic inches as the quantity of air equally inhaled 

 id exhaled about 20 times in a minute, it will follow that a full-grown per- 



* Priestley had before shown that nitrogen is absorbed. See Phil. Trans. 1790, p. 106. 



