LIGHT AND LIFE. 295 



century that so rich a subject of study began to attract serious experi- 

 mental research ; and such are the difficulties of this grand and complex 

 problem, that its solution is only partly revealed, in spite of a long 

 series of attempts. Great deficiencies remain to be supplied, and many 

 vaguely-known points to be cleared up, nor has an efibrt even been 

 made as yet to systematize all the groups of results gained. The latter 

 task we propose to attempt here, with the purpose of showing by a 

 remarkable instance the manner of evolving knowledge through the 

 power of the experimental method, the sequent, cumulative, and mu- 

 tually-supporting character of w r ell-conducted experiments, and their 

 endless wealth of instruction ; in a w r ord, the process adopted by emi- 

 nent men in the great art of wresting her secrets from living Nature. 



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 atmos- 

 phere carbonic-acid gas. They decompose this gas into carbon, 

 which they assimilate, and oxygen, which they reject. Now, this phe- 

 nomenon, which is the vegetable's mode of respiration, can only be 

 accomplished 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 philoso- 

 phy, only in consequence of a serious affection of his sight, was the first 

 to detect this joint work, about the middle of the eighteenth century. 

 He remarked 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 immersed in w T ater 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 enclosure, 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 impura 

 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, Ingenhousz, succeeded in explaining thia 

 apparent contradiction. " I had but just begun these experiments,'* 

 says that skilful naturalist, " when a most interesting scene revealed 

 itself to my eyes : I observed that not only do plants have the power 

 of clearing impure air in six days or longer, as Priestley's experiments 

 seem to point out, but that they discharge this important duty in a 

 few hours, and in the most thorough way ; that this singular operation 

 is not due at all to vegetation, but to the effect of sunlight ; that it does 



