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.
The manner in which we find the actinic power influencing electrical action, also shows us that the equilibrium of forces is continued through all the great principles of nature. If a galvanic arrangement is made, by which small quantities of metals may be slowly precipitated at one of the poles in the dark, and a similar arrangement be exposed to sunshine, it will be found that no metal is deposited: the sun’s rays have interfered with the decomposing power of the electrical current. At the same time we learn, that by throwing a beam of light upon a plate of copper which forms one of a galvanic pair, whilst it is under the influence of an acidulated solution, an additional excitation takes place, and the galvanometer will indicate the passage of an increased current of electricity. These two dissimilar actions appear enigmatical; but they may, there is no doubt, receive some solution from the influence of different rays on the contrary poles of the battery. One thing is quite evident,—electricity suffers a disturbance of one order, by light; and an excitement of another by its associated principles in the sunbeam. If a yellow glass is interposed between the galvanic arrangement and the sun, the electro-chemical precipitation goes on in the same manner as it would in perfect darkness, and no extra excitement is produced upon the plates of the battery. From this it would appear that actinism and not light is to be regarded as the disturbing power.[128] It has already been shown that yellow media possess the power of stopping back the chemical agent.
We have already, detailed many of the peculiarities of the different varieties of Phosphori, which would seem to be the result of light. Phosphorescence is probably excited by those rays which produce no direct effect upon the eye. If we spread sulphuret of calcium upon paper, and expose it to the action of the solar spectrum, it is found to glow (in the dark) only over those spaces occupied by the violet rays and the ordinarily dark rays beyond them; proving that the excitation necessary to the development of the phenomena of phosphorescence is due to a class of rays distinct from the true light-giving principle, and more nearly allied to that principle or power which sets up chemical decomposition. Whether the fluorescent rays, before mentioned, which are found so abundantly over the space which produces the greatest phosphorescent effect, are active in producing the phenomena, is as yet an unsolved problem.
Vision and colour, calorific action, chemical change, molecular disturbance, electrical phenomena, and phosphorescent excitation, all, each one with a strange duality, are connected with the sunbeam.
We find, when we receive solar spectra upon iodized plates, or on several kinds of photographic paper, that a line, over which no action takes place, is preserved at the top and bottom of the impressed image, and in many cases along the sides also. The only way in which this can be accounted for, as the spectrum represents the sun in a distorted form, is by supposing that rays come from the edges of the sun of a different character from those which proceed from the centre of that orb.[129]