94 DECOMPOSITION OF CARBONIC ACID, ETC. 
few hours the tube becomes half full. On detonation with 
hydrogen, this gas proves to be rich in oxygen. 
Prof. D. made some attempts to discover how much oxy- 
gen could in this way be evolved from known quantities of 
bicarbonate of soda ; supposing it probable that the second 
atom of carbonic acid being removed and decomposed, the 
process would cease: however, the results of his experiments 
indicated that the supposition he had formed was not correct. 
The process is not limited to the removal and decomposition 
of the second atom, but goes forward, the first itself being 
in like manner decomposed. From this it would seem that 
carbonate of soda itself should be decomposed ; and experi- 
ment verified the conclusion: for on using that salt instead 
of the bicarbonate, the evolution of oxygen went on pre- 
cisely in the same way. 
As in these experiments a solid salt is decomposed, it is 
obvious that the function by which the leaves accomplish 
this is very different from that of respiration. It is not re- 
spiration, but a true digestion. 
In the same way Prof. D. found that all kinds of soluble 
carbonates and several other organic salts, such as bitartrate 
of potash, citrate of soda, succinate of ammonia, &c., would 
emit oxygen. 
It thus seems that the decay of some nitrogenized body 
in the leaf is essential to the digestive action of plants. 
At this stage of the inquiry, a remarkable analogy ap- 
pears between the function of digestion in animals and the 
same function in plants. Liebig has shown how, from the 
transformation of the tissues of the stomach itself, food be- 
comes acted upon, and is turned into chyle, an obscure 
species of fermentation brought about by the decay of ni- 
trogenized bodies. So in like manner in plants ; the disso- 
lution of a nitrogenized body brings about the assimilation 
of carbon. The facts seem to indicate, that the primary 
action of the light is not upon the carbonic acid, but upon 
