Researches in Vegetable Physiology. 149 



only ones which can become the cause of the decomposition 

 of carbonic acid. The rays endowed with the most consider- 

 able illuminating-power (the yellow rays) exert of themselves 

 an influence equal to that of all the others taken together. 

 The most refrangible rays possess only a much less marked 

 action. To each spectral colour there belongs a certain degree 

 of activity in the phenomenon of assimilation, a degree which 

 remains the same whether the rays act isolatedly upon plants, 

 or whether their action is combined." 



To arrive at the greatest possible exactitude, M. PfefFer 

 passed over the different methods which consist either in 

 counting the bubbles of gas or in measuring the quantities of 

 gas which have escaped from a plant vegetating under water. 

 He adopted the method of M. Boussingault, who made his 

 plants vegetate in a closed vessel, the atmosphere of which 

 contained known quantities of carbonic acid. As coloured 

 liquids he employed chromate of potash, ammoniacal oxide of 

 copper, aniline red, orselline, aniline violet, and chlorophyl, 

 and also, in order to observe the effect of the obscure heat- 

 rays, a very concentrated solution of iodine in sulphide of 

 carbon. We cannot, however, describe the apparatus and 

 experiments ; for these details we must refer the reader to the 

 memoir itself. 



We may say, only, that from the commencement of his in- 

 vestigation M. Pfeffer foresaw that the effects of the two 

 halves of the spectrum separated by the chromate of potash 

 and the ammoniacal oxide of copper represented, when taken 

 together, a total nearly equal to the action of white light. 

 This was already a great step made towards the idea of the 

 predominant action of the luminous intensity. It is in con- 

 sequence of this observation that M. PfefFer, by employing 

 sometimes monochromatic liquids, sometimes liquids which 

 only excluded one or two spectral colours, has succeeded in 

 nearly determining the assimilant power of each ray. If in 

 white light chlorophyl decomposes 100 parts of carbonic 

 acid, the isolated rays give the following numbers : — 



Red and orange 32*1 



Yellow 46-1 



Green 15*0 



Blue, indigo, violet 7'6 



Total. . 100-8 



We may therefore truly say that the action of the com- 

 bined light represents the sum of the partial actions which the 

 isolated rays would exert. The knowledge of these numbers 



