HISTORY OF THE DISCOVERY. '2S.i 



is tlie immediate aoent in ministeriu2: to the nutrition of 

 plants. In 1843, by causing plants to effect the decom- 

 position of carbonic acid in the prismatic spectrum, I found 

 that the yellow is by for the most effective, the relative 

 power of the various colours being as follows : yellow, 

 green, orange, red, blue, indigo, violet. My experiments 

 on the production of hydrochloric acid by the direct union 

 of chlorine and hydrogen under the influence of light, both 

 solar and artificial, conclusively establish the fact that the 

 primary condition essential for the chemical action of light 

 is the absorption of some particular ray. If the physical con- 

 dition of substances otherwise easily decomposable is such 

 that they transmit light mthout absorbing any, no chemi- 

 cal chano;e ensues in them, and the same in cases of combina- 

 tion. 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, remem- 

 ber 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. -j- Learn to look at the sun 



* Draper : Hitman Phi/tiiologii : 185(5, p. 461. 



f "Blumen, Blatter, Fiiichte, sind also aus Luft gewebte Kinder des 

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



U 



