HISTORY OF THE DISCOVERY. 247 



by the direct union of clilorine and hydrogen under 

 the influence of light, both solar and artificial, conclu- 

 sively establish the fact that the primary condition 

 essential for the chemical action of light is the absorp- 

 tion of some j^articular ray. If the physical condition 

 of substances otherwise easily decomposable is such that 

 they transmit light without absorbing any, no chemical 

 chano'e ensues in them, and the same in cases of com- 

 binatiou. Thus oxygen and hydrogen cannot be made 

 to unite, even by the most intense radiation, because 

 neither of them exerts any absorptive action ; but 

 chlorine and hydrogen unite with energy, because the 

 chlorine absorbs the indigo ray." * 



Such has been the history of this partial withdrawal 

 of the veil which hides the mysterious connection of 

 light with life. And now, reader, as you ramble through 

 the cornfields, and see the shadows running over them, 

 remember that every wandering cloud which floats in 

 the blue deep, retards the vital activity of every plant 

 on which its shadows fall. Look on all flowers, fruits, 

 and leaves as air- woven children of the light, f Learn 

 to look at the sun with other eyes, and not to think of 

 it as remote in space, but nearly and momentarily con- 

 nected with us and all living things. Astronomy may 

 measure the mighty distance which separates us from 

 that blazing pivot of life ; but Biology throw^s a lumin- 

 ous arch, w^hich spans those millions upon millions of 



* Draper : Human Physiology : 1856, p. 461. 



f " Blumen, Blatter, Friichte, sind also aus Luft gewebte Kinder 

 des Lichts." — MoLESCHOTT : Licht und Leben : 1856, p. 29. 



