232 SEA-SIDE STUDIES. 



liglit aiul organisation is soon told. It was not suspected 

 until 1771, when Priestley discovered that the plant gave 

 out an air which was capable of maintaining combus- 

 tion. He allowed a burning candle to extinguish itself in 

 a closed vessel, into which he subsequently introduced a 

 living plant ; and in ten days this plant had so altered the 

 condition of the contained air, that the candle once more 

 ignited in it. Many a schoolboy can now explain this, 

 which was then a splentlid discovery, and opened the path 

 whereon, three years later, Priestley laid a foundation-stone 

 of modern chemistry. Progress in science is at times 

 unaccountably slow. For fifteen years had Europe been 

 acquainted with the fiict that growing plants set free 

 " oxygen," as we call it ; but no further step was taken, 

 till Ingenhouss showed that this oxygen could only be 

 developed by plants ivhen 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 sunlight 

 enables the leaf to liberate oxygen from the carbonic acid of 

 the air. He proved that sun-heat alone would not suffice ; 

 sunlight was the agent at work. Living physiologists have 

 even separated the particular ray of sunlight which exerts 

 the intensest effect. Professor Draper was tlie first to show 

 this. In his recent work he says : " Since the sunlight is 

 composed of many differently coloured rays, and different 

 principles, it becomes an interesting inquiry which of these 



• Sennebiek : .S'ur C Influence de la LuniQre solaire jwitr metamorphoser CA ir 

 Axe en A ir fiur par la Vegelaiion, 1783. 



