CONNEXION BETWEEN ABSORPTION AND CHEMICAL ACTION. 91 



369. From these theoretical considerations it would appear that, in the case of chlo- 

 rine and hydrogen, the latter gas is wholly passive ; the chlorine, being acted upon, 

 absorbing the chemical rays, is thrown into such a condition that union takes place 

 with the hydrogen. 



370. A remarkable consequence follows from these views. If the reason that oxy- 

 gen and hydrogen cannot form water under the influence of the sunlight be due to the 

 circumstance that neither of those gases can absorb tithonic rays, but are perfectly 

 transparent and colourless, and the reason that chlorine and hydrogen at once form mu- 

 riatic acid, be due to the absorbent capacity of the chlorine, it results that when a mix- 

 ture of these latter gases intercepts a ray, the absorbent action upon that ray should 

 not be greater than that of the chlorine alone, and even not more than one half, because 

 of the diluted state in which the chlorine is presented. But this is the same conclu- 

 sion to which we have previously arrived by direct experiment (321). 



371. The views which have been given in this chapter serve to show that chemical 

 action is the uniform result of absorption ; but the converse of the proposition does not 

 hold good ; absorption is not necessarily attended by chemical action. Nevertheless, 

 it is attended with a certain effect. Even in the case of an elementary, and therefore 

 unchangeable substance like chlorine, a disposition or capacity for union is communi- 

 cated. Chlorine that has been exposed to the sun unites with hydrogen more readily 

 than chlorine which has been made and kept in the dark (Ap., CH. XVIII.), 



CHAPTER XI. 



ON THE MODE OF ACTION OF LIGHT IN DIRECTING THE DIGESTION OF PLANTS. 



CONTENTS : Connexion between Absorption and Chemical Action. Radiant Matter is 

 absorbed in producing different Effects. Reappearance of the Force expended. 

 Laws of Preliminary Absorption and Definite Action observed by Plants. Increased 

 Rapidity of Vegetation implies increased Brilliancy of the Incident Light. The 

 Sun probably a Periodic Star. Description of the Mode of Action of Light and 

 Radiant Heat on Leaves. 



372. WE are now ready to take up the consideration of the question proposed in 

 Chapter II. : In what manner does light act in directing the digestive function of 

 plants 1 



373. From various phenomena exhibited by radiant heat, M. MELLONI has developed 

 the doctrine of invisible calorific coloration, those of latent heat and absorption having 

 been established many years ago. In the same manner, from phenomena connected 

 with the tithonic rays, we have developed for them analogous doctrines in the prece- 

 ding chapters. As respects light, the views which, for the other imponderables, are 

 only ideal or imaginary, for it become certain ; because our organs of vision inform us 



