Dr Alison on the Principle of Vital Affinity. 139 



tery, by which we are not able to separate the oxygen from 

 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 Chemistry^ p. 134.) 



Next let us observe the extent to which this energetic 

 power is exercised by living plants. Perhaps the most ac- 

 curate idea of it may be formed from attending to the state- 

 ment of Theodore de Saussure, that on a mean of 54 obser- 

 vations 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 exist- 

 ing in the atmosphere was found to be diminished very nearly 

 10 per cent, in a few hours of every day ; and for this dimi- 

 nution 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 affi- 

 nities, 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 effi^ct 

 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, within 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 uniformly ex- 

 hibit 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 



* See Macaire's Memoir of Theodore de Sausaure, in Jameson's Edinburgh 

 I'hilosophical Journal, vol. xl., p. 31 (Jan. 1846.). 



