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 arc 

 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 



