THE MECHANISM OF LIFE 547 



cell is pre-eminently the energy of sunlight. Almost no chemical 

 energy is introduced into the plant ; the chemical substances 

 from which the plant constructs its living substance, namely, 

 carbonic acid, water and the salts dissolved in it, are compounds 

 which in this form contain almost no chemical potential. These 

 compounds are transferred to substances possessing chemical 

 potential only by the introduction of light mediated by the 

 activity of chlorophyll in the green plant-cell. The affinities of 

 carbon and oxygen, e.g., can be made available only by the splitting- 

 up of carbonic acid, COj, into carbon and oxygen. Energy is 

 consumed in this process, and the amount required for it is supplied 

 from the energy introduced by light into the plant-cell. It 

 has, therefore, been said that all life is derived in direct descent 

 from sunlight ; and thus in a certain sense an exact scientific 

 background is given to the ancient poetic worship of light and the 

 sun by Asiatic and American races. But sober scientific con- 

 sideration forces us to modify the above statement. If the idea 

 that the sun's light-rays are the energy from which all the energy 

 of the living world in the last instance is derived, may be expressed 

 at all in this general form, it is true only for the conditions now 

 prevailing upon the earth's surface. If we go back to the times 

 when the living substance first appeared upon the earth, we shall 

 doubtless be obliged to turn to chemical energy as that form of 

 energy that was first introduced into the living substance. Of 

 course the living substance of the present day, like all substance, 

 is finally derived, together with its energy, from the sun, for the 

 earth is only a part of the sun's mass thrown off; but light can 

 hardly be considered directly as that form of energy which effected 

 upon the cooling earth the construction of those compounds, con- 

 taining energy in the potential form, that are termed living sub- 

 stance. In reality, at present it is not the light that directly 

 accomplishes the splitting-up of the carbonic acid and the combi- 

 nation of the atoms of carbon, hydrogen and oxygen into the first 

 product of assimilation, starch. This idea, which perhaps has been 

 suggested by an inexact mode of expression, is wholly incorrect. 

 In reality, it is only the chemical energy of certain compounds of 

 the chlorophyll-bodies in the green plant-cell that does this. The 

 energy of the light-rays alone can never split up carbonic acid, 

 quite apart from the coupling of the atoms of carbon with those of 

 hydrogen and oxygen into starch molecules. Light is indispensable 

 only in so far as it is that form of energy which favours the 

 rearrangement of the atoms in certain compounds of the chloro- 

 phyll-bodies, so that these atoms are able to enter into chemical 

 relation with those of carbonic acid and thus decompose the latter. 

 The energy of the light-rays, therefore, is first transformed into 

 chemical energy, and it is the latter in the chlorophyll-bodies that 

 effects the splitting-up of carbonic acid and therewith inaugurates 



N N 2 



