Observatio7is on Liebig^s " Organic Chemistrij.'" 121 



part of the functions of the leaves are to separate nitrogen in 

 the form of ammonia, and this may be part of the effect in pro- 

 portioning the elements of the sap so as to produce flower buds. 

 The professor's reasonings on the subject of nitrogen are very 

 profound, and in advance, and should be well studied. 



The action of light, he says, stimulates the power of the leaves 

 to assimilate, or prepare for assimilation, the food ; and presup- 

 poses a power and capability in light, to which the most powerful 

 chemical combination cannot be compared. The strongest gal- 

 vanic battery cannot separate the oxygen from the carbonic acid ; 

 and though chlorine, by its affinity for hydrogen, forming muriatic 

 acid, will separate the oxygen from water, under the influence of 

 light, yet it cannot be considered as at all equalling the power 

 and energy with which a leaf separated from a plant, and thus 

 in a mutilated state, decomposes the carbonic acid. De Candolle 

 says, the leaf loses the power of decomposing carbonic acid 

 when the plant dies; a separated leaf should be approaching 

 that state ; cuttings and leaves will live some time after separa- 

 tion, but the living powers cannot be so active. 



The direct rays of the sun, he says, are most powerful, but dif- 

 fused light has the same effect, though not so intense and rapid. 

 The presence of nitrogen is not needed in the decomposition of 

 carbonic acid. The carbon is needed in the formation of the dif- 

 ferent substances containing carbon ; hence it is evident, he says, 

 the quantity and quality of the substances generated by the vital 

 processes must vary according to the food that is supplied, and 

 the developement of the individual organs will proceed according 

 as the substances necessary for their developement are furnished. 

 The quality will also depend on the quantity, as the action of 

 heat and light in the leaves will separate the oxygen needed to 

 be removed from acid mucilaginous substances, in converting 

 them into sugar and starch, more easily from a small than a 

 large quantity ; if the quantity of sap is great, it will require 

 more action to prepare it. In this process heat seems greatly to 

 augment the action of light. When the leaves of celery are 

 shut out from light, the oxygen is preserved even though heat 

 is present ; and the bitter poisonous principle in which hydrogen 

 predominates, formerly generated when light was present to the 

 leaves by the deoxidation of the sap, is now converted into 

 sugar. In like manner, if the tubers of potatoes, which are 

 part of the stem, are exposed to the light, they secrete a bitter 

 poisonous principle, similar to that of the stems and leaves, by 

 the deoxidation. Though heat alone will not cause the deoxi- 

 dation, yet we see it assists it greatly ; as, in the instance of fruit 

 trees on walls, we find the separation of the oxygen, and ripen- 

 ing of the acid fruit, proceed with a great deal more vigour 

 than in standard trees, though fully exposed to the light. So 



