DIFFUSION OF GASES IN THE LUNGS. 835 



scale enabled Graham to determine the true numerical expression or law of 

 this diffusive power or energy of gases, viz., that the rate of diffusion of any 

 gas, if dry and pure, is inversely as the square root of its density or specific 

 gravity. Graham showed, moreover, that this diffusion takes place through 

 narrow tubes and through porous substances, according to the same law, pro- 

 vided that the gases be dry and chemically indifferent to each other, and to 

 the substance of the porous septum. On the other hand, when films of India- 

 rubber or of shellac, moist animal membranes, or even soap-bubbles, are em- 

 ployed as the septa interposed between any two gases, diffusion still takes place, 

 but then not according to the above-mentioned law, but under modifications 

 dependent on the relative solubility of either gas in the interposed septum. 

 Lastly, experiments made by Draper on gases in a state of solution, show that 

 these still manifest mutually diffusive tendencies, although not according to 

 Graham's law of their simple diffusion in a dry state. This moist diffusion of 

 gases has been termed false gaseous diffusion. 



Both simple and spurious diffusion occur in aerial respiration per- 

 formed by lungs or air-sacs, but the latter only, in aquatic respiration, 

 performed by gills or moist surfaces. 



The breathing air in calm respiration, about 20 cubic inches, amounts 

 to only |th of the reserve and residual air together, 180 cubic inches, 

 which are ordinarily retained in the lungs (see p. 819). Even in 

 active respiration, it would only amount to about Jth, viz., 45 cubic 

 inches. Hence, so small a displacement of the air in the lungs, 

 at each inspiration and expiration, cannot directly influence the air 

 contained in the remote air-cells, especially as the bronchial tubes con- 

 stantly increase in their total capacity, from the trachea to the air- 

 sacs. The simple diffusion of gases here comes into play ; for since, 

 as we shall presently see, the last portion of air expelled in a long 

 expiration is richer in carbonic acid than the first portion, it is proba- 

 ble that the residual air, which is never expelled from the lungs, be- 

 comes increasingly richer in carbonic acid gas, and therefore poorer in 

 oxygen, in the direction of the air-cells ; hence the diffusion of oxy- 

 gen must take place from the larger bronchi, to which the pure air 

 gains access, towards the air-cells ; whilst carbonic acid diffuses itself 

 in the opposite direction, from the air-cells towards the larger air-tubes. 

 The respiratory movements doubtless continually change the air in 

 the lungs, and, as it were, partially ventilate the air-passages ; but the 

 energy and rapidity of the diffusive process, and its incessant opera- 

 tion, supplement their effects. The diffusive process is accelerated by- 

 differences of temperature between any two gases, a condition con- 

 stantly operating in respiration. Moreover, the pulmonary exhala- 

 tion, which, in the air-cells and smaller air-tubes, exists in the form of 

 vapor, likewise has a similar tendency to diffuse into the drier air in 

 the larger passages. That this diffusion of carbonic acid gas actually 

 occurs in the lungs, may be shown by steadily holding the breath, with 

 the open mouth kept in communication with a bag, or other reservoir, 

 holding a known volume of atmospheric air, when this latter is soon 

 found to contain a readily appreciable quantity of carbonic acid. In 

 apparent death or trance, when the respiratory movements are sus^ 

 pended, a minimum respiratory interchange of gases may thus take 

 place, just sufficient to prevent the extinction of life. In the deepest 



