THE ACTION OF LIGHT 577 



the reciprocal of the thickness of the medium which sufficed to reduce light of a 

 particular wave length to one-tenth of its value. The manner in which it is 

 derived from Lambert's and Beer's laws and its application to spectro-photometry 

 are described in the text. 



The phenomena of resonance play a large part in the mechanism of photo- 

 chemical reactions. If the vibration period of a molecule coincides with that of 

 any of the light waves falling upon it, the molecule will be set into resonant 

 agitation by means of the light energy absorbed. This vibration usually leads to 

 chemical change. Luther's theory of the mechanism of the resonance process and 

 its consequence is given in the text. 



Photo-chemical reactions do not obey the law of mass action, because their rate 

 is controlled by the amount of light energy absorbed per unit time. 



In order to set a photo-chemical reaction in train, a certain amount of light 

 energy is absorbed, so that the first stage is always associated with the taking up 

 of energy. The later stages may be either similar to that of the chlorophyll 

 system, in which the continuance of the reaction depends on a continual supply of 

 light energy and results in a storage of energy, or the reaction may be one which 

 proceeds of itself with evolution of energy, but is accelerated by means of a 

 catalyst produced by light. The catalyst may disappear in the reaction itself and 

 require to be formed by the continual action of light, as in the case of hydrogen 

 and chlorine. Or it may be more or less permanent and continue to act after the 

 light is removed. Another class of reactions is that of the coupled reactions, in 

 which the products of the light reaction are used up at once in another reaction 

 with loss of energy. 



Optical sensitisers belong to the class of catalytic light reactions. A system, 

 insensitive to light of a particular wave length, can be made sensitive to it if a dye 

 be present which absorbs this wave length. Since the light is absorbed by the 

 sensitiser, it must act by means of the changes produced in this, resulting in the 

 formation of some substance, frequently active oxygen, or some catalyst, which 

 acts on the system which is by itself insensitive to the particular light in question. 



The Bunsen-Boscoe law states that the product of the intensity of the light 

 and its time of action produces a constant effect, so that intensity and time of 

 action can mutually make up for each other. This law is replaced by an ex- 

 ponential ratio when a photographic plate is developed after exposure. This fact 

 seems to depend on the intervention of an adsorption phenomenon in the process 

 of development. 



The phenomena of fluorescence and phosphorescence are shown to be, probably, 

 cases of photo-chemical reactions with storage of light energy, which is given off 

 again afterwards. 



Chemi-luminescence is the name applied to the emission of light in a reaction 

 at a temperature much below that corresponding to the wave length of the light 

 given off, if the system had merely been raised in temperature by the application 

 of heat. It consists in the direct conversion of chemical energy into light, without 

 passing through heat, and is the converse of those photo-chemical reactions in which 

 light energy is converted directly into chemical energy, as in the chlorophyll 

 system of the green leaf. 



The reactions brought about by the chloroplasts of the green leaf result in the 

 storage of a large amount of energy derived from the sun. Carbon dioxide and 

 water are changed into starch and oxygen. 



Although the presence of the pigment chlorophyll is indispensable for the 

 occurrence of the reaction, owing to its absorption of the light energy, there is no 

 satisfactory evidence to show that carbon dioxide can be reduced by the pigment 

 alone. 



Chlorophyll is the ester of a complex acid, consisting of pyrrol derivatives 

 linked to magnesium. This acid is combined with a monatomic hydrocarbon 

 alcohol, phytol, with twenty carbon atoms. 



37 



