376 M. H. Sainte-Claire Deville on Affinity and Heat. 



it begins to move, and contributes to the increase of the plant. 

 It is a solution so diluted with carbonic acid and organic or 

 mineral substances drawn from the seed or from the soil, that 

 its molecules may be considered absolutely free, or separated 

 by the latent heat which is there accumulated. When this solu- 

 tion reaches the parenchyma of the leaves, where both its con- 

 centration and its alteration on contact with the elements of the 

 air take place, it may be said that all molecular equilibria are 

 successively possible ; and if circulation raises them to concen- 

 tration or to combination at a given moment, all the elements of 

 carbonic acid, of water, and of the mineral principles which the 

 sap contains may group themselves according to a formula de- 

 termined previously by the rapidity of the circulation, the nature 

 of the leaves, and the physical circumstances necessary for the 

 life of the plant. In this manner we can probably account for 

 the diversity which the act of vegetation imprints on the nature 

 of the products it forms with the elements of water, of carbonic 

 acid, of ammonia, and of some minerals. 



It is also in the heat stored up by solution that we must seek 

 the principal element of the decomposition by vegetables of car- 

 bonic acid into carbon and oxygen — a phenomenon regarding 

 which we must confess our total ignorance. 



A most remarkable experiment which M. Berthelot has made, 

 by placing together carbonic oxide, water, and potash, belongs to 

 actions of this kind. 



Carbonic oxide dissolves in potash, and absorbs during its 

 extension* in the liquid a certain number of thermal units 

 beyond that which the loss of vis viva by contraction doubtless 

 allows it to retain at the moment of its liquefaction. This solu- 

 tion, which takes place in very small quantity at a time, owing 

 to the slight solubility of the gas, is really a considerable exten- 

 sion, which, thanks to dissociation by diffusion, gives to the mole- 

 cules of carbonic oxide the heat necessary for entering into direct 

 combination with the elements of potash. In this reaction (it 

 is moreover effected with the slowness which characterizes all 

 operations where solution is necessary when the solubility is 

 small) the heat is fixed which formic acid needs for its existence. 

 It is the origin of this heat taken from the solution by a com- 

 bination effected between bodies in the nascent state, and which 

 will be disengaged in the form of sensible heat when formic acid 

 is decomposed by spongy platinum, as has been done by Berthelot, 

 or when formic acid is burned, as was done by Favre and Silber- 

 mann some time ago. 



* A given weight of carbonic oxide has a far less density in its aqueous 

 solution than in the atmosphere formed by the gas itself on the surface of 

 the solvent. 



