170 DR ALISON'S OBSERVATIONS ON 



let US next observe the intensity of the action by which the carbonic acid is 

 there decomposed, the carbon attached to the elements of the water, and the 

 oxygen set free. " This is done by a power," says IjIEbig, " to which the strongest 

 chemical action cannot be compared. The best idea of it may be formed by con- 

 sidering, that it surpasses in power the strongest galvanic battery, by which Ave 

 ai*e not able to separate the oxygen fi'om carbonic acid. The affinity of chlorine 

 for hydrogen, and its power to decompose water, under the influence of light, and 

 set its oxygen at liberty, cannot be considered as nearly equalling the power and 

 energy with which a leaf, separated from a plant, decomposes the carbonic acid 

 which it absorbs." — Organic Chemishy, p. 134. 



Next let us observe the extent to which this energetic power is exercised by 

 living plants. Perhaps the most accurate idea of it may be formed from attend- 

 ino- to the statement of Theodore de Saussure, that on a mean of 54 observations 

 made in a country district, the proportion of carbonic acid in the atmosphere 

 during the night was to its proportion in the day-time as 432 to 398, i. e., the 

 carbonic acid existing in the atmosphere was found to be diminished very nearly 

 10 per cent, in a few hours of every day; and for this diminution we know no 

 cause, except that this power of the green parts of vegetables, of decomposing the 

 carbonic acid of the atmosphere, is exercised only under the influence of light.* 



Now if a power of this extraordinary energy and extensive operation, and 

 acting in so very simple a manner, were really to be regarded as depending 

 only on ordinary chemical affinities, exerted under peculiar conditions, it might 

 surely be expected, that the chemist might so regulate the conditions under which 

 he might bring together carbonic acid, air, and water, as to exhibit some traces 

 of this power, and effect some decomposition of the carbonic acid and evolution 

 of oxygen. But we know, not only that this cannot be done, but that when air, 

 water, and carbonic acid, are introduced into the very same vegetable cells, with- 

 in half an hour after they have exhibited this phenomenon, at the same spot, 

 under the same light, and at the same temperature, they will not only fail to 

 exhibit the same change, but will unifonnly exhibit the very reverse, i. e., the 

 absorption of oxygen and the formation and evolution of carbonic acid. 



Nay, we know that it is only in certain cells of the living vegetable, that this 

 peculiar chemical change, under the action of light, is effected ; the same fluid, 

 introduced into cells composed of the same material in the parts of fructification, 

 undergoes no such change ; but, on the contrary, gives occasion only to the reverse 

 process, the absorption of ox3'gen and evolution of carbonic acid.f 



Then it is to be remembered, that this complete inversion of ordinary che- 

 mical affinities, in the case of the living plant, is only one of several cases to 



* See Macaire's Memoir of Theodore de Saussure, in Edinburgh Philosophical Journal, vol. xl. 

 p. 31. (.Ian. 18^6,) 



^ Theououe de Saussure, in Edinburgh Philosophical Journal, vol. xl. pp. 22, 23. 



