THE ACTIONS OF FOKCES ON OEGANIC MATTEE. 31 



readily decomposed by light, are those in which there 

 is a marked contrast between the atomic weights of the 

 constituents, and probably therefore a marked contrast 

 between the rapidities of their vibrations. The circumstance, 

 too, that different chemical compounds are decomposed or 

 modified in different parts of the spectrum, implies that there 

 is a relation between special orders of undulations and special 

 orders of composite atoms — doubtless a correspondence 

 between the rates of these undulations and the rates of 

 oscillation which some of the components of such atoms 

 will assume. Strong confirmation of this view may 



be drawn from the decomposing actions of those longer 

 ethereal waves which we perceive as heat. On contemplating 

 the whole series of binary compounds, we see that the ele- 

 ments which are most remote in their atomic weights, as 

 hydrogen and the noble metals, will not combine at all : their 

 vibrations are so unlike that they cannot keep together 

 under any conditions of temperature. If again we look at a 

 smaller group, as the metallic oxides, we see that whereas 

 those metals that have atoms nearest in weight to the atoms 

 of oxygen, cannot be separated from oxygen by 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 temperatures ; 

 and those which differ from it most widely, combine with it 

 very reluctantly, and yield it up if exposed to thermal undu- 

 lations of moderate intensity. And here indeed, remem- 

 bering 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 infiuence 

 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 by light in the leaves of plants. 

 Among the several elements concerned, there are wide differ- 



