LIGHT AND LIFE. 103 



I. 



Plants gain their nourishment by the absorption through 

 their roots of certain substances from the soil, and by the 

 decomposition through their green portions, of a particular 

 gas contained in the atmosphere carbonic-acid gas. They 

 decompose this gas into carbon, which they assimilate, and 

 oxygen, which they reject. Now, this phenomenon, which 

 is the vegetable's mode of respiration, can only be accom- 

 plished with the assistance of solar light. 



Charles Bonnet, of Geneva, who began his career by 

 experimenting on plants, and left this attractive subject, to 

 devote himself to philosophy, only in consequence of a seri- 

 ous affection of his sight, was the first to detect this joint 

 work, about the middle of the eighteenth century. He re- 

 marked that vegetables grow vertically, and tend toward 

 the sun, in whatever position the seed may have been 

 planted in the earth. He proved the generality of the fact 

 that, in dark places, plants always turn toward the point 

 whence light comes. He discovered, too, that plants im- 

 mersed in water release bubbles of gas under the influence 

 of sunlight. In 1771, Priestley, in England, tried another 

 experiment. He let a candle burn in a confined space till 

 the light went out, that is, until the contained air grew unfit 

 for combustion. Then he placed the green parts of a fresh 

 plant in the inclosure, and at the end of ten days the air 

 had become sufficiently purified to permit the relighting of 

 the candle. Thus he proved that plants replace gas made 

 impure by combustion with a combustible gas ; but he also 

 observed that at certain times the reverse phenomenon seems 

 to result. Ten years later, the Dutch physician, Ingen- 

 housz, succeeded in explaining this apparent contradiction. 

 "I had but just begun these experiments," says that skill- 

 ful naturalist, " when a most interesting scene revealed it-' 

 self to my eyes : I observed that not only do plants have 

 the power of clearing impure air in six days or longer, as 



