the Carbon exhaled by the Lungs. 431 



it does not increase the quantity of carbon evolved, or rather 

 that with starch there is a very small increase, and with fat a 

 corresponding decrease. Hence on this theory these substances 

 have no action ; and yet they must be the chief source of the 

 supply of carbon to the system. It is true that by this test, 

 viz. the absence of effect in increasing the evolution of carbon, 

 we cannot prove that these substances thus taken are ultimately 

 resolved into their elements ; but we know well that they do 

 become transformed and take a share in maintaining the evolution 

 of carbon, for example, to the minimum line above mentioned. 



3rd. There are numerous substances contained in food, which, 

 when taken alone, cause the evolution of more carbon than they 

 themselves contain. Such are tea, gluten, and casein, — the first 

 causing a maximum increase of from 1| to 2| grains, and the two 

 latter of 1 grain of carbonic acid per minute, the duration of the 

 increase existing from one to two hours. 



4th. Other substances, and those which are the richest in cai'- 

 bon, contain much more carbon than is evolved by the increase 

 which they occasion. Such are all cereals and sugars, both of 

 which cause a maximum increase of upwards of 2 grains of car- 

 bonic acid per minute, with a duration of the increase of 3 hours 

 from the former and 1 to 1| hour from the latter. 



Hence if it be assumed that in any case the increased evolu- 

 tion of carbon is due directly to the transformation of the food 

 which gave rise to it, it fails to account for nearly every phseno- 

 menon connected with the action of foods, and is destitute of 

 proof. 



The second theory, or that which assumes that evolution of 

 carbonic acid is due to indirect or intermediate influences acting 

 upon the blood, is supported by numerous facts. Under this 

 head I refer to the hypothesis of Baron Liebig, who has shown 

 that soda is the base which has the greatest capacity for the ab- 

 sorption of carbonic acid, and to the facts that the carbonic acid 

 thus combined, may be set free either by the introduction of a free 

 acid into the blood, or by the diminution of carbonic acid in the 

 air on the lung-surface of the capillaries. Hence food, during 

 its transformation, but before its flnal resolution into carbonic 

 acid, may act indirectly by disengaging the carbonic acid then 

 and previously existing in the blood; and whatever will cause 

 increased depth of inspiration, may induce the more rapid disen- 

 gagement of carbonic acid from the blood by lessening the 

 amount of that gas existing in the air-vesicles of the lung. 

 AVithout assuming that this theory is true in its entirety, the 

 following facts offer great support to this or similar views. 



1. Vierordt has shown that increased depth of inspiration 

 causes increased evolution of carbonic acid; 



