LIGHT AND LIFE. 103 
IE 
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. Jn 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 plated 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. 
*“*T 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 
