EFFECT OF LIGHT ON CHEMICAL COMBINATION. 105 



power the strongest galvanic battery, with which we are not able 

 to separate the oxygen from carbonic acid. The affinity of 

 chlorine for hydrogen, and its power of decomposing water under 

 the influence of light, and of setting at liberty its oxygen, cannot 

 be considered as at all equalling the power and energy with 

 which a leaf separated from a plant decomposes the carbonic 

 acid absorbed by it. 



In living plants and in their seeds, there exists a peculiar 

 power different from all other causes of increase of mass. This 

 power, however, only shows itself in action when aided by the 

 influence of heat or of light. In spring, when the heat of the sun 

 penetrates the earth, the asparagus may put forth shoots of 

 many feet in length quite independently of the action of light. 

 But the constituents of these shoots were formerly constituents 

 of the roots. A conversion of pre-existing compounds into new 

 products, and their assumption of new forms, can proceed with- 

 out light, although not without heat. But this is not a true in- 

 crease of mass, or an increase in the quantity of carbon. The 

 latter process only takes place under the influence of light. 



The common opinion that only the direct solar rays can effect 

 the decomposition of carbonic acid in the leaves of plants, and 

 that reflected or diffused light does not possess this property, is 

 wholly an error, for exactly the same constituents are generated 

 in a number of plants, whether the direct rays of the sun fall 

 upon them, or whether they grow in the shade. They require 

 light, and indeed sun-light, but it is not necessary that the direct 

 rays of the sun should reach them. Their functions certainly 

 proceed with greater intensity and rapidity in sunshine than in 

 the diffused light of day ; but there is nothing more in this than 

 the similar action which light exercises on ordinary chemical 

 combinations ; it merely accelerates in a greater or less degree 

 the action already subsisting. 



Thus chlorine and hydrogen combining form muriatic acid. 

 This combination is effected in a few hours in common daylight, 

 but it ensues instantly, with a violent explosion, under exposure 

 to the direct solar rays, whilst not the slightest change in the 

 two gases takes place in perfect darkness. When the oil formed 

 from defiant gas is exposed in a vessel with chlorine gas to the 

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