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 Liste, “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 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 accurate idea of it may be formed from attend- 
ing to the statement of THEopoRE DE Saussure, that on a mean of 54 observations 
made ina 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, 7.¢., 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 uniformly exhibit the very reverse, 7. ¢., 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 oxygen and evolution of carbonic acid.+ . 
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 Macarre’s Memoir of THEoporE DE SaussuRE, in Edinburgh Philosophical Journal, vol. xl. 
p. 3l. (Jan. 1846,) 
+ TxHeEopore bE Saussure, in Edinburgh Philosophical Journal, vol. xl. pp. 22, 23. 

