246 SEA-SIDE STUDIES. 



a schoolboy can now explain this, which was then a 

 splendid discover}^, and opened the path whereon, three 

 years later, Priestley laid a fonndation-stone of modern 

 chemistry. Progress in science is at times unaccountably 

 slow. For fifteen years had Europe been acquainted 

 with the fact that growing plants set free " oxygen," 

 as we call it ; but no further step was taken, till Ingen- 

 houss showed that this oxygen could only be developed 

 by plants vjhen in sunlight. Neither he, nor any one 

 else, suspected whence came this oxygen ; that was a 

 mystery for another ten years, when Sennebier's work* 

 gave to science the simple and pregnant fact, that sun- 

 light enables the leaf to liberate oxygen from the car- 

 bonic acid of the air. He proved that sun-heat alone 

 would not suffice; sunliajht was the ao-ent at work. 

 Living physiologists have even separated the particular 

 ray of sunlight which exerts the intensest effect. Pro- 

 fessor Draper was the first to show this. In his recent 

 work he says : " Since the sunlight is composed of many 

 diflTereutly coloured rays, and difi"erent principles, it 

 becomes an interesting inquiry which of these is the 

 immediate airent in ministerino; to the nutrition of 

 plants. In 1843, by causing plants to efi'ect the de- 

 composition of carbonic acid in the prismatic spectrum, 

 I found that the yellow is by far the most eff"ective, 

 the relative power of the various colours being as fol- 

 lows: yellow, green, orauge, red, blue, indigo, violet. 

 My experiments on the production of hydrochloric acid 



* Sennebier : Sar t Influence de la Lumiere solaire jwur metamor- 

 liliosertAirfixeeiiAirjmrparla Vefjetation. 1783. 



