THE EFFECTS OF HEAT AND RADIANT ENERGY 113 



If we wish to study the physiological effects of ether waves, we may 

 therefore confine ourselves to the waves of shorter length. Among 

 these waves those are especially interesting for us whose length is 0.8 p 

 and less, inasmuch as these waves affect our retina, and produce those 

 chemical effects in green plants which make assimilation in these plants 

 possible. 



Among the various known effects of these waves two are of impor- 

 tance for us; namely, the photochemical effect and the radiation 

 pressure. The latter seems to be of great importance as far as cos- 

 mical phenomena are concerned, as Arrhenius has shown; but I do 

 not believe that they play any role in life phenomena, as Radl seems 

 to assume,* who believes they are responsible for the heliotropic 

 effects of light. This view is, as I believe, contradicted by the fact 

 that radiation pressure is independent of the wave length, while 

 the heliotropic effects are eminently a function of the wave length. 

 This latter influences, however, the photochemical effects, and for this 

 reason it seems advisable to consider the possibility that the biological 

 effects of light are indirectly chemical effects. It seems that every 

 chemical reaction which is influenced by ether waves at all can be 

 influenced only by waves of a definite, limited period. As in this case 

 radiating energy is transformed into chemical energy, the light waves 

 can have no effect unless they are absorbed. We find indeed that in 

 all cases only such light waves produce a chemical or biological action 

 as are absorbed ; but the reverse statement, that wherever in an organ- 

 ism an absorption of light occurs (e.g. in pigment spots) a biological 

 effect must be produced, is not correct. 



In order to give an idea of the possible chemical effects of light, a 

 few instances may be quoted. Ultraviolet rays cause the formation 

 of ozone from the oxygen of the air, as can be beautifully demonstrated 

 with the aid of the Heraeus mercury quartz lamp. According to Vogel, 

 violet rays cause the oxidation of guaiacum and give it a blue color, 

 while red rays reduce it and make it appear yellow. The oxidizing 

 and reducing effects of light seem to be of special physiological im- 

 portance. Thus Duclaux attributes the well-known sterilizing effect 

 of light upon bacterial cultures partly, at least, to the formation of acids 

 which are produced by the light in the nutritive medium. He has 

 shown that fats are oxidized and hydrolized under the influence of 

 light, and that the acid thus formed acts antiseptically. According 

 to the same author, sugars are oxidized in an alkaline medium by light. 

 Hydrogenperoxide is also found among the products formed under 

 the influence of light in culture media. 



* Radl, Untersuchungen liber den Phototropismus der T/iiere, Leipzig, 1903. 



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