LIGHT AND LIFE. 297 



sudden activity after it has passed. By intercepting the solar "beam 

 with a screen, the alternations of quickness and slowness in the pro- 

 duction of gas-bubbles may be very plainly seen, according as the 

 plant receives the rays or not. Water-plants show other interesting 

 peculiarities. Diffused light has no power to excite the production of 

 carbonic acid, unless the phenomenon has been first called forth by 

 direct sunlight. Still further, the solar influence having once been ap- 

 plied, the evolution of carbonic acid continues even in darkness. The 

 vegetable keeps up at night its mode of breathing by day. The living 

 force of solar light, therefore, can be fixed and stored away in living 

 plants, as Van Tieghem, the discoverer of this curious property, very 

 well remarks, to act afterward in complete darkness, and exhaust it- 

 self by slow degrees, through transformation into equivalent chemical 

 energy. It appears to lodge itself in phosphorescent sulphur, to reap- 

 pear under the form of less intense radiations ; it hoards itself up in 

 paper, starch, and porcelain, to come forth anew, after a greater or less 

 lapse of time, through its action on the salts of silver. The peculiarity 

 residing in these green cells of vegetables, then, is not an isolated one : 

 it is a special instance of the general property, inherent in many bod- 

 ies, of retaining, within their mass, in some unknown form, a part of 

 the vibrations that fall upon them, and of preserving them through 

 transformation, to be afterward emitted, either in the state of luminous 

 radiations, or in the condition of chemical or mechanical energy. The 

 great principle of the transformation of forces thus holds good in the 

 vegetable kingdom. And we end with the remark that these facts of 

 persistent activity, called out by an initial excitement, lend support to 

 the idea that living forces hold a close connection with the molecular 

 structure of bodies, and may even be the determinate expression of 

 that structure. "We cannot conceive manifold energy in a mathemati- 

 cal and irreducible atom ; but in a molecule, made up of a certain num- 

 ber of atoms, we can fancy dynamic figures of a very complex order. 



We have thus far regarded only the action of white light, the effect 

 of the totality of rays sent us by the sun ; but this light is not simple. 

 It is composed of a great number of radiations, of distinct colors and 

 properties. When white light is decomposed by the prism, we obtain 

 seven groups of visible rays, of unequal refractive power, violet, indi- 

 go, blue, green, yellow, orange, and red. The spectrum or ribbon of 

 colors thus obtained widens and spreads out by invisible radiations. 

 Beyond the red, there exist radiations of dark heat, or calorific. rays, 

 and, outside of the violet, radiations which are called chemical or ultra- 

 violet rays. The first affect the thermometer, the last occasion ener- 

 getic reactions in chemical compounds. What is their influence upon 

 vegetation ? Does solar light act by its colored rays, its heat-rays, or 

 its chemical rays ? 



The question has been subjected to many important experiments, 

 and is, perhaps, not yet determined. Daubeny, in 1836, was the first 



