CHEMICAL RAYS. 243 



question compels us to supplement, if not materially to 

 qualify, this conception. It is a most remarkable fact that 

 the waves which have thus far been found most effectual in 

 shaking- asunder the atoms of compound molecules are 

 those of least mechanical power. Billows, to use a strong 

 comparison, are incompetent to produce effects which are 

 readily produced by ripples. It is, for example, the violet 

 and ultra-violet rays of the sun that are most effectual in 

 producing these chemical decompositions ; and, compared 

 with the red and ultra-red solar rays, the energy of these 

 " chemical rays " is infinitesimal. This energy would 

 probably in some cases have to be multiplied by millions 

 to bring it up to that of the ultra-red rays : and still the 

 latter are powerless where the smaller waves are potent. 

 We here observe a remarkable similarity between the be- 

 havior of chemical molecules and that of the human retina. 

 The energy transmitted to the eye from a candle-flame half 

 a mile distant is more than sufficient to inform conscious- 

 ness ; while waves of a different period, possessing twenty 

 thousand million times this energy, have been suffered to 

 impinge upon my own retina, with an absolute unconscious- 

 ness of any effect whatever mechanical, physiological, 

 chemical, or thermal. 



If, then, the power of these smaller waves to unlock the 

 bonds of chemical union be not a result of their strength, it 

 must be, as in the case of vision, a result of their periods 

 of recurrence. But how are we to figure this action ? The 

 shock of a single wave produces no more than an infini- 

 tesimal effect upon an atom or a molecule. To produce a 

 larger effect, the motion must accumulate, and for wave- 

 impulses to accumulate, they must arrive in periods iden- 

 tical with the periods of vibration of the atoms on which 

 they impinge. In this case each successive wave finds the 

 atom in a position which enables that wave to add its shock 

 to the sum of the shocks of its predecessors. The effect is 



