the Carbon exhaled by the Lungs. 433 



6. I have found great variation in the action of acids and 

 alkalies at various periods of the day and year in reference to this 

 subject ; but commonly acids somewhat increased the evolution 

 of carbonic acid, and alkalies lessened it. (It may be remarked 

 that lactic acid never gave an increase.) This general action, 

 as well as the variations observed, are explicable on this theory, 

 since the one will increase the disengagement, and the other the 

 absorption of carbonic acid ; and both will be dependent upon 

 the existing relation between the free acid and base in the blood. 



Hence this theory of the indirect elimination of carbonic acid 

 from the blood is supported by and explains important facts ; 

 but it leaves untouched the source whence the carbon is directly 

 supplied to the blood. It would of course follow as an inference, 

 that this removal of carbonic acid from the blood would give 

 temporarily a greater capacity to the blood for the reception 

 of a new supply; and it is possible that this might tend to the 

 increased resolution of material into carbonic acid, and thus 

 carry us a step backward in the process ; but proof is yet wanting. 

 This, however, leads us to the third theory, viz. an increase in 

 the changes proceeding in the vital tissues. 



There are numerous facts which support this view. Thus: — 



1. Exertion, which must imply the production of muscular 

 power, and therefore the waste of that tissue, causes instantaneous 

 increase in the evolution of carbonic acid ; and that increase is, 

 within the limits of the strength and power of endurance of the 

 individual, joffn' passu with the increase of exertion. This is of 

 course quite independent of either the direct or indirect action of 

 food, is so constant that during the working day it is almost ever 

 acting, and so powerful, that in running at full speed or working 

 the treadwheel it causes an increase of seven times the quantity 

 at rest. It is always accompanied both by increased depth and 

 frequency of respiration, and thereby it alters the relation of the 

 carbonic acid on the two sides of the capillary membrane, in ac- 

 cordance with the second theory ; but this increase is a conse- 

 quence of the exertion, and is due to the anterior cause of the 

 increased action of the non-respiratory muscles. Hence, whilst 

 exertion may act on the second, it certainly acts on the third 

 theory. 



2. The cause of the inci'eased evolution of carbonic acid from 

 a low temperature is not so easily explained. The power of 

 absorbing carbonic acid by the blood varies with the temperature, 

 and the air inspired varies in temperature ; but since the tempe- 

 rature of the intimate tissue of the lungs and of the blood varies 

 but little, it is difficult to see how the influence of low temperature 

 can be exerted. It is not explained by variation in the amount 

 of exertion made at different seasons ; for when at perfect rest, 



Phil. May. S. 4. Vol. 18. No. 123. Dec. 1859. 2 P 



