34 THE DATA OF BIOLOGY. 



heat, even when it is joined by a powerful collateral affinity; 

 those metals which differ more widely from oxygen in their 

 atomic weights, can be de-oxidized by carbon at high tem 

 peratures; and those which differ from it most widely com 

 bine with it very reluctantly, and yield it up if exposed to 

 thermal undulations of moderate intensity. Here indeed, 

 remembering the relations among the atomic weights in the 

 two cases, may we not suspect a close analogy between the 

 de-oxidation of a metallic oxide by carbon under the influence 

 of the longer ethereal waves, and the de-carbonization of 

 carbonic acid by hydrogen under the influence of the shorter 

 ethereal waves? 



These conceptions help us to some dim notion of the mode 

 in which changes are wrought in light in the leaves of plants. 

 Among the several elements concerned, there are wide differ 

 ences in molecular mobility, and probably in the rates of 

 molecular vibration. Each is combined with one of the others, 

 but is capable of forming various combinations with the rest. 

 And they are severally in presence of a complex compound 

 into which they all enter, and which is ready to assimilate 

 with itself the new compound molecules they form. Certain 

 of the ethereal waves falling on them when thus arranged, 

 cause a detachment of some of the combined atoms and a 

 union of the rest. And the conclusion suggested is that 

 the induced vibrations among the various atoms as at first 

 arranged, are so incongruous as to produce instability, and 

 to give collateral affinities the power to work a rearrange 

 ment which, though less stable under other conditions, 

 is more stable in the presence of these particular undula 

 tions. There seems, indeed, no choice but to conceive 

 the matter thus. An atom united with one for which it has 

 a strong affinity, has to be transferred to another for which 

 it has a weaker affinity. This transfer implies motion. The 

 motion is given by the waves of a medium that is relatively 

 imponderable. No one wave of this imponderable medium 

 can give the requisite motion to this atom of ponderable 



