THE P II AS IS OF FORCE. 177 



out ; and of such it has been ascertained by Boussingault, that the 

 same aggregate amount of light and heat is required by each kind 

 for the sustentation of its whole term of activity from germination 

 to the maturation of its seed, under whatever latitude it be grown ; 

 that term being so uniformly abbreviated by an exaltation, and 

 protracted by a depression, in the intensity of these forces, as to 

 show that its rate of life must stand in a direct ratio to them. 



We have already seen that the influence of light is exerted in 

 providing the material for vegetable growth by a quasi-chemical 

 action ; and it is capable of proof by direct experiment, that, 

 Cftcris paribus, the quantity of carbonic acid decomposed by a 

 plant in a given time is proportional to the amount of light that 

 has fallen upon it. There is no reason to suppose that light acts 

 upon more than the surface, or that it lias any direct concern with 

 the internal operations of growth and development. On the 

 contrary, we find that at one most important epoch, that of ger 

 mination, these processes are most actively carried on in the 

 dark ; it being only when all the store of nutriment laid up in the 

 seed has been exhausted, and when the young plant is beginning 

 to be dependent upon that which it obtains for itself, that the 

 influence of light becomes requisite. On the other hand, the rate 

 of germination is so closely dependent, as every maltster knows, 

 upon the degree of heat to which the seed is exposed, that it is 

 capable of being exactly regulated by an increase or a diminution 

 of the temperature ; and thus we are led to regard heat as the 

 force by which the vegetable germ is enabled to appropriate the 

 nutriment prepared for it, and to organize this into living tissue. 

 Such a view, however, is by no means equivalent to the assertion 

 that heat is itself the &quot; vital principle,&quot; or the organizing force. 

 We do not say that heat is electricity, because the heating of a 

 certain combination of metals produces an electric current through 

 them ; nor do we say that heat is mechanical force, because by 

 boiling water we generate an elastic vapour. In each of these 

 instances the character of the force is changed ; and so it is here. 

 The living organism is the medium of transmutation, like the bis 

 muth and antimony in the first case, or like the water in the 

 second ; and its special peculiarity is, that it converts the heat, 

 not only into vital force generally, but into that peculiar form of 



