PLANTS AND THE ATMOSPHERE. 419 



rapidly exhaust the oxygen -which they replace by carbonic acid and soon die, not 

 by a poisonous effect of the gas, but from want of respiratory nourishment. 



Having thus recalled these facts, I shall now proceed to describe a famous 

 experiment, which, without our Imowledge, plants are unobtrusively performing 

 among us ; whicli is accomplished upon an immense scale ; and which may 

 rightly be considered one of the most essential phenomena in the world : an 

 experiment otherwise so simple that every one is able and must I:>e willing to 

 repeat it. In order to succeed, take a fresh and sound leafy stem of the aquatic 

 plants which are to be found growing submerged in ponds and rivers, place it in 

 a white glass decanter filled with spring water or better still, with diluted 

 Seltzer water which, as we are aware, contains a great proportion of carbonic 

 acid in solution. Having corked the filled decanter, invert it, so as to introduce 

 the neck into a vessel filled with water. The cork may then be withdrawn, the 

 water remaining undisturbed and continuing to fill the inverted decanter. The 

 apparatus being thus prepared, remove it to an open place where it may receive 

 the rays of the sun. 



As soon as the light strikes directly upon the leaves of the immersed plant, they 

 will be seen covered with a multitude of bubbles which rapidly increase in size, 

 unite, and rise to the top of the vessel where they accumulate. "Whenever the 

 light is intercepted by an opaque screen, this disengagement ceases, and by alter- 

 nately covering the apparatus with light or shade, even at a distance, the current 

 may be reproduced or stopped, at will. After several Hours of continuous 

 action, the decanter is filled with this gas. It resembles air but has not the pro- 

 perties of air, for if the vessel being returned to its original position, a slender wax 

 taper, just extinguished and still retaining at the end of the wick some red points 

 be immediately introduced, it will, at once, rekindle and continue- to burn with 

 unusual brilliancy. This gas, then, is not air but oxygen. The same experiment 

 may be reproduced with aerial plants, and, in order not to change their habitual 

 condition, they should be exposed to the sun under glass receivers filled with 

 carbonic acid. After a day this gas will disappear and will be found to be 

 replaced by nearly pure oxygen. Whatever the plant may be, whatever the 

 experimental pi'ocess, the action always remains identical, and the explanation of 

 this important fact is evident. The green parts of plants decompose carbonic 

 acid; they extract from it the carbon which they retain, and reject the oxygen 

 which they return to the atmosphere. In darkness and during the night, their 

 action changes. Far from absorbing carbonic acid they exhale it ; but this 

 nightly reaction being inferior to the diurnal action, plants finally accomplish a 

 part diametrically opposed to that performed by animals. They destroy the 

 carbonic acid which they form, they generate anew the oxygen which they 

 absorb and thus reproduce the organic matter which they consume. 



At the sight of so perspicuous an experiment and of so simple an explanation 

 it seems that scientific men dught to have discovered them from the very first. 

 It would, however, be a strange delusion to believe that such was the case. IsTo 

 great discovery is made without cost to humanity. At first, all is obscurity and 

 impotence ; it is only after long investigation that, amid much hesitation, a glimpse 

 of some scattered truth is caught, and, until the moment when a serene light 



