The admirer of nature may copy her arrangements with the utmost fidelity. Every modulation of the landscape, each projecting rock or beetling tor--the sinuous river in its rapid flow--the meandering stream, “gliding like happiness away;” and the spreading plains over which are scattered the homes of honest industry and domestic peace, intermingled with the towers of those humble temples in which simple-hearted piety delights to “bow the head and bend their knee;” these, all of these, may, by the sunbeam which illuminates the whole, be faithfully pencilled upon our chemical preparations. Our art enables us to do more even than this; we have but to present our sensitive tablet to the moon, and she, by her own light, prints her mountains and her valleys, and indicates with all truth the physical conditions of her surface. Any reference to the _chemical agency_ of LIGHT--_the luminous rays_ as distinguished from the _chemical and calorific rays_--has been avoided until we came to the consideration of this particular question of chemical change. Upon organic compounds, as, for instance, upon the colouring matter of leaves and flowers, _light_ does exert a chemical power: and it is found that vegetable colours are bleached, not by rays of their own colour, but by those which are _complementary_ to them. A red dye fades under the influence of a green ray, and a yellow under that of a violet one, much more speedily than when exposed to rays of any other colour; and this, it must be remembered, is due to the coloured ray itself, and not to any _actinic_ power masked, as it were, behind the colour, as is generally believed.[125] It was long a question whether the decomposition of carbonic acid by plants was due to the luminous or the chemical rays. It is now clearly established that the luminous rays are the most active in producing this effect; which they do indirectly, by exciting the vital powers of the organized structures. Therefore we would refer this phenomenon of gaseous decomposition to a vital power quickened by luminous excitement.[126] We have already noticed some chemical phenomena due to heat, particularly those experiments of Count Rumford’s, which appeared to him to prove that the chemical agency of the sun’s rays was due to its calorific power. Certain chemical phenomena, we know, may be produced by thermic action; but the only variety of thermo-chemical action which connects itself immediately with the solar radiations, belongs to a class of rays to which the name of _Parathermic_ has been given, and to which the scorching, as it is called, of plants, the browning of the autumnal leaves, and the ripening of fruits, appear to be due.[127] When we come to the consideration of those physical phenomena which belong to the growth of plants, all these peculiarities of solar action must be attended to in detail.