LIGHT AND VEGETATION 103 



For the chemist, each term in this equation represents a 

 gram-molecule, i.e., a well-determined mass of matter: 

 18 grammes of water, 44 grammes of carbon dioxide, 

 30 grammes of formol, and 32 grammes of oxygen. Thermo- 

 chemistry would enable us to calculate exactly the quantity 

 of energy necessary to effect the reaction if we knew the 

 substance that was really formed (the substance that we have 

 supposed to be formol). It happens that every reasonable 

 hypothesis on the nature of this substance leads to results 

 very close to 112,000 calories; we can therefore complete 

 our equation, without knowing exactly the product of the 

 reaction, by stating that such is the energy provided by 

 Hght. 



Our intention now is to explain the initial phase of a photo- 

 chemical reaction. The action of Ught came to be understood 

 by the physical study of extremely simple cases when our 

 knowledge of the atomic structure of matter, and of the 

 quantic structure of hght, permitted the analysis of the ele- 

 mentary phenomena of interaction between matter and 

 radiation. 



We must first show how the process of a chemical reaction 

 is generally represented. Take a concrete example — the 

 combination of chlorine and hydrogen, with the formation 

 of hydrogen chloride. The real phenomena are extremely 

 complex, but suppose that influences such as the presence of 

 water vapour, the nature of the walls of the receptacle, etc., 

 do not play any part. 



In the dark, the two gases mix and remain in contact 

 without combining. This is a very common case; even very 

 exothermic reactions are not produced when the con- 

 stituents exist alone, but only when an "excitation factor" or a 

 "catalyst" is added to facihtate the passage from the initial 

 state of the uncombined substances to the more stable final 

 state in which the combination has been accomplished. 



Similarly, billiard balls at rest on a table do not fall to the 

 ground, although their fall would correspond to a diminution 

 of their energy and therefore to a more stable final state 



