244 FRAGMENTS OF SCIENCE. 



mechanically the same as that due to the timed impulses 

 of a boy upon a swing. The single tick of a clock has no 

 appreciable effect upon the unvibrating and equally long 

 pendulum of a distant clock ; but a succession of ticks, each 

 of which adds, at the proper moment, its infinitesimal push 

 to the sum of the pushes preceding it, will, as a matter of 

 fact, set the second clock going. So likewise a single puff 

 of air against the prong of a heavy tuning-fork produces 

 no sensible motion, and, consequently, no audible sound ; 

 but a succession of puffs, which follow each other in periods 

 identical with the tuning-fork's period of vibration, will 

 render the fork sonorous. I think the chemical action of 

 light is to be regarded in this way. Fact and reason point 

 to the conclusion that it is the heaping up of motion on the 

 atoms, in consequence of their synchronism with the shorter 

 waves, that causes them to part company. This I take to 

 be the mechanical cause of these decompositions which are 

 effected by the waves of ether. 



And now let us return to that faint cloudiness already 

 mentioned, from which, as from a germ, these considerations 

 and speculations have sprung. It has been long known that 

 light effected the decomposition of a certain number of 

 bodies. The transparent iodide of ethyl, or of methyl, for 

 example, becomes brown and opaque on exposure to light, 

 through the discharge of its iodine. The art of photography 

 is founded on the chemical actions of light ; so that it is 

 well known that the effects for which the foregoing theoretic 

 considerations would have prepared us, are not only proba- 

 ble, but actual. 



But the method employed in the experiments in which 

 the cloudiness above referred to was observed, and which 

 consists simply in offering the vapors of volatile substances 

 to the action of light, enables us not only to give such ex- 

 periments a beautfiul form, but also to give a great exten- 

 sion to the operations of light, or rather of radiant force, as 



