sudden activity after it has passed. By intercepting the solar beam with a screen, the alternations of quickness and slowness in the production 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 applied, 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 itself by slow degrees, through transformation into equivalent chemical energy. It appears to lodge itself in phosphorescent sulphur, to reappear 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 bodies, 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 mathematical and irreducible atom; but in a molecule, made up of a certain number 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, indigo, 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 ultraviolet rays. The first affect the thermometer, the last occasion energetic 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
