THE ACTIONS OF FORCES ON ORGANIC MATTER. 33 



stand how the Sun's raj's can effect chemical decompositions. 

 If the members of a diatomic molecule stand so related to the 

 undulations falling on them, that one is thrown into a state of 

 increased oscillation and the other not; it is manifest that 

 there must arise a tendency towards the dislocation of the 

 two — a tendency which may or may not take effect, accord- 

 ing to the weakness or strength of their union, and according 

 to the presence or absence of collateral affinities. This in- 

 ference is in harmony with several significant facts. Dr. 

 Draper remarks that " among metallic substances (com- 

 pounds) those first detected to be changed by light, such as sil- 

 ver, gold, mercury, lead, have all high atomic weights; and 

 such as sodium and potassium, the atomic weights of which 

 are low, appeared to be less changeable.^^ As here interpreted, 

 the fact specified amounts to this; that the compounds most 

 readily decomposed by light, are those in which there is a 

 marked contrast between the atomic weights of the consti- 

 tuents, 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 rela- 

 tion between special orders of undulations and special orders 

 of molecules — doubtless a correspondence between the rates of 

 these undulations and the rates of oscillation which some of 

 the components of such molecules 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 diatomic com- 

 pounds, we see that the elements which. are most remote in 

 their atomic weights, as hydrogen and the noble metals gen- 

 erally, will not combine at all, or do so with great diffi- 

 culty : 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 which have atoms nearest in weight to 

 the atoms of oxygen, cannot be separated from oxygen by 



