342 Mr Carpenter on the Differences of the Laws 



effects its conversion into sugar, just as the chemist may do by 

 heat and sulphuric acid : this diastase is prenously stored up 

 in contact with the embryo, and when the vitaHty of the latter 

 is excited by its appropriate stimuli, it is carried into the sub- 

 stance of the albumen. Diastase may be easily obtained from 

 the tuber of the potato, where it surrounds the buds ; and 

 when the chemist employs it as a reagent, its effect on starch 

 corresponds with that which it produces in the living plant, 

 and with that of the purely chemical operation of sulphuric 

 acid. Another example is afforded by the function of respira- 

 tion,* which, although not yet perfectly elucidated, seems al- 

 most entirely under the control of physical and chemical laws. 

 The object of this function is the excretion of superfluous car- 

 bon from the system, in a manner calculated to give rise to the 

 development of heat. We shall not enter into the question at 

 present most keenly debated, with regard to the form in which 

 carbon exists in venous blood. This appears to have been re- 

 cently decided by the experiments of Magnus ; but there can 

 be no question that, when that fluid is exposed to the air through 

 the medium of a thin membrane, an interchange of ingredients 

 takes place, carbonic acid being removed and oxygen absorbed. 



The removal of carbonic acid is proved by the experiments of Edwards 

 on respiration in hydrogen ; the absor[)tion of oxygen (which cannot be de- 

 tected free in arterial blood) by the predominance which often occurs of the 

 quantity of oxygen which has disappeared over that of the carbonic acid 

 which replaces it. 



A constant absorption and exhalation of nitrogen also ap- 

 pears to take place. Now, all these facts are explicable upon 

 the known laws of the mutual diffusion of gases. The most 

 evidentof these changes take place after the removalof blood from 

 the animal, if it be exposed either directly or through a membra- 

 nous septum to the contact of oxygen ; and we can scarcely, 

 therefore, view them in any other light than as chemical. But 

 these changes, like others which result immediately from the 

 relations between organized beings and the external world, al- 

 though themselves resulting from physical laws, require for 

 their continuance the assistance of actions dependent upon vital 



* We use the term in its most general sense, denoting the interchange of 

 ingredients between the air and circulating fluid. 



