THE ACTIONS OF FORCES ON ORGANIC MATTER. 31 



but small.* On plants, however, the solar rays that 



produce in us the impression of yellow, are the immediate 

 agents of those molecular changes through which are hourly 

 accumulated the materials for further growth. Experiments 

 have shown that when the Sun shines on living leaves, they 

 begin to exhale oxygen and to accumulate carbon and hydro- 

 gen — results which are traced to the decomposition, by the 

 solar rays, of the carbonic acid and water absorbed. It is now 

 an accepted conclusion that, by the help of certain classes 

 of the ethereal undulations penetrating their leaves, plants 

 are enabled to separate from the associated oxygen those two 

 elements of which their tissues are chiefly built up. 



This transformation of ethereal undulations into certain 

 molecular re-arrangements of an unstable kind, on the over- 

 throw of which the stored-up forces are liberated in new 

 forms, is a process that underlies all organic phenomena. It 

 will therefore be well if we pause a moment to consider 

 whether any proximate interpretation of it is possible. Re- 

 searches in molecular physics give us some clue to its nature. 



The elements of the problem are these : — The atoms f of 

 several ponderable matters exist in combination : those which 

 are combined having strong affinities, but having also affin- 

 ities less strong for some of the surrounding atoms that are 

 otherwise combined. The atoms thus united, and thus mixed 

 among others with which they are capable of uniting, are 

 exposed to the undulations of a medium that is so rare as to 

 seem imponderable. These undulations are of numerous 

 kinds : they differ greatly in their lengths, or in the fre- 



* The increase of respiration found to result from the presence of light, is 

 probably an indirect effect. It is most likely due to the reception of more 

 vivid impressions through the eyes, and to the consequent nervous stimulation. 

 Brij^ht light is associated in our experience with many of our greatest oiit- 

 door pleasures, and its presence partially arouses the consciousness of them, 

 with the concomitant raised vital functions. 



f To exclude confusion it may be well here to say that the word " atom " 

 is, as before explained, used as the name for a unit of a substance at present 

 undccomposed ; while the word " molecule" is used as the name for a unit of 

 a substance known to be compound. 



