Prof. Graham on the Law of the Diffusion of Gases, 355 



piration, about 20 cubic inches, or J-jth of the contents are 

 thrown out, from the application of a general pressure to 

 the whole. But it is evident, that these twenty cubic inches 

 will be the twenty cubic inches nearest the outlet, or the con- 

 tents of the larger tubes. The contents of the second-sized 

 tubes will advance at the same time into the largest tubes, but 

 no further, and will recede again into their original deposi- 

 tories on the next inspiration, which will fill the larger tubes 

 with fresh air; which identical quantity will again be ex- 

 pelled in the next expiration. This illustration is perhaps too 

 strongly stated ; but it is evident that, in ordinary respiration, 

 the slight mechanical compression will have little or no effect 

 in emptying the most distant tubes, or the ultimate air-cells, 

 of their contents. The bulk of the air, also, is not altered du- 

 ring respiration, although, for a quantity of oxygen, carbonic 

 acid gas is substituted. This substitution, which is the great 

 end of respiration, undoubtedly takes place most abundantly 

 in the minute and distant air-cells, which present the largest 

 surface to the blood ; and the carbonic acid there produced, 

 must be moved along the smaller tubes by the diffusion pro- 

 cess, (which we know to be extremely energetic, and also in- 

 evitable,) till it is thrown into the larger tubes, from which it 

 can be expelled by the ordinary action of respiration. But 

 the action of diffusion is always twofold : at the same time that 

 carbonic acid is being carried outward from the air-cells, oxy- 

 gen is carried inward in exchange, and thus the necessary cir- 

 culation kept up throughout the whole lungs. 



Further, by a forced expiration, from 160 to 178 cubic 

 inches may be expelled, after which, there still remain in the 

 lungs about 120 cubic inches, which are not under the con- 

 trol of the respiratory action. 



There can be no doubt that much of this quantity occupies 

 constantly and permanently the most minute tubes and air- 

 cells, for it can scarcely be withdrawn by means of the air- 

 pump. Now, the question has arisen, how these ultimate tubes 

 and air-cells are so powerfully inflated; for they are not dis- 

 tended by the action of muscular fibre, of which they are 

 known to be destitute. This state of distention must be highly 

 useful, by exposing surface; and the law of diffusion enables 

 us to account for it. The heavy carbonic acid which these 

 minute cells may contain, is not merely exchanged for oxygen, 

 but for a larger volume of oxygen, in the proportion of the 

 diffusion- volumes of carbonic acid and oxygen; namely, 81 

 carbonic acid are replaced by 95 oxygen. The resistance to 

 passage through the most minute tubes, is overcome by the 



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