LIGHT A VITAL AGENCY. 245 



food can be assimilated from witliout. Nay, if the 

 experiment be carefully conducted, you will find that 

 your plant in darkness, in spite of appareiit growth, 

 has really lost some of its substance, instead of increas- 

 ing it ; weighing less, when dried, than the dry seed 

 from which it issued. Science has proved that it is in 

 light, and in light alone, that plants deoxidise carbonic 

 acid — setting free the oxygen, which can then be 

 breathed by animals, and in thus setting free the oxy- 

 gen, releasing the carbon, which nourishes the tissues of 

 the plant. It was thought (and is still printed in 

 many text-books) that the green parts caused the lib- 

 eration of oxygen in light ; but Mulder corrects this, 

 saying that the parts do not liberate the oxygen be- 

 cause they are green, but become green in the process.'" 

 Eear the plant in darkness, and its leaves will be pale ; 

 bring it into sunlight, and these pale leaves instantly 

 decompose carbonic acid, and assume a green tint. 



The history of our knowledge of the relation be- 

 tween lio'ht and oro-anisation is soon told. It was not 



CD O 



suspected until 1771, when Priestley discovered that 

 the plant gave out an air which was capable of main- 

 tainino; combustion. He allowed a burnino; candle to 

 extinguish itself in a closed vessel, into which he sub- 

 sequently 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 



* Mulder: Versuch einer Physiol. Chemie, 1851. A translation of 

 this valuable work was published under the auspices of the late Pro- 

 fessor Johnston. 



