now proved the existence of these rays of Actinism in the most unquestionable manner. It is at present impossible to explain the real nature of the Actinic force; we must rest satisfied with its definition as "that power in the sun's ray which produces the chemical changes taking place in bodies exposed to the light." These delicate yet potent rays fall, like the sunlight in which they exist, upon every portion of the surface of the landscape, and together with those of heat and light, they produce effects of a most important kind. The towering trees of yonder forest, as well as the humble moss which clusters on their bark, owe their health and vigour-nay, their very existence-to the Actinic rays. Take away these from them, and they become sickly and feeble, and die. It is almost more surprising to discover that these rays exert a most powerful influence upon inanimate bodies. It is found to be impossible to expose any solid substance, whatsoever its nature, to the sun's rays without its undergoing some change, in consequence of the operation of these Actinic rays on the surface. Wonderful thought! the sunbeam cannot even impinge upon a plate of the hardest steel without leaving a trace of its passage behind. Every object in the outer world is affected by this agency. Those rugged cliffs and those tall and frowning mountains are, for every hour that the sunlight strikes them, undergoing a destructive change; and the most extensive effects would soon be produced, were it not that a beautiful remedy has been provided by which the injurious results that would otherwise follow are entirely obviated. If the world had not, like man, its stated times of rest, it would soon undergo the most serious changes, the end of which would be, undoubtedly, an entire alteration of every object on its surface. During the silent hours of the night, however, it has been found that all these effects of the solar ray pass off, and all bodies restore themselves again to their original condition. It is not therefore only to man or the animate world in general, or to vegetation, that night and gentle sleep come as "Nature's sweet restorers'; the great earth must rest likewise. These fields and yonder hills sleep and become restored and refreshed, equally with the living and moving beings on their surface. Night is precious alike to all-in truth, it is indispensable. The beautiful contrivance employed for portraittaking, the Daguerreotype, exhibits to us in a singularly striking manner the potency and rapidity of the action of the Actinic rays of the sunbeam. In a darkened chamber, generally in a little box appropriately fitted, the rays of light proceeding from the sitter's figure are collected by means of a lens, and are caused to fall upon a silvered plate, which has been prepared by exposing it to the vapour of iodine. In a second of time, the most faithful picture of the person is fixed upon the metal surface, in lines which years cannot efface. By means of a Camera Obscura (dark closet or chamber), Mr. Talbot succeeded in producing the most exquisite sun-pictures, in which all the varying tones of light and shade, and every line of the scene, is exhibited with an accuracy to which no painter has attained, or could attain. This invention is called the Talbotype. Thus, the researches of modern science have enabled us to press the sunbeam into our service, as an artist more speedy in execution and more admirably accurate in its productions than the most skilful of men. Nature's own pencil is now employed to depict itself; the fairest landscape imprints its own image upon the enduring surfaces of metal and paper; the most minute points of detail are thus indelibly preserved to us, and the wanderer in a foreign land needs little exertion of his own to store his portfolio with pictures drawn by the sun, which, on his return, may often serve to bring to his recollection scenes and objects then far distant. At Greenwich a great number of the various astronomical observations are self-registered, by employing the Actinic power of light. The details are too complicated to be easily understood; this application, however, is chiefly made, in the case of the magnetic observations, by little mirrors placed upon the needles which reflect the light of a lamp on to a sheet of prepared paper. Thus, when the needles moye, they cause the reflected light also to move a certain distance on the paper; and wherever this light falls, it leaves its mark in the discoloration that instantly takes place. It has also been proposed to copy objects in the microscope, by casting the image on prepared paper. This would be highly useful. This delightful art is called Photography, that is, Light-drawing; but it would be preferable to style it Heliography, or Sun-drawing, since it is the Actinic rays, not strictly the Light or luminous rays of the sunbeam, which produce it. Hitherto, we can be said to have perfectly succeeded only in producing pictures of one uniform tint; but some singular experiments have been made, and are still in the course of prosecution, which seem to indicate that, in time, it will be even possible to produce perfect pictures, each object being represented in its natural colours. This would indeed be a triumph in the science of light. The warm and pleasant sunshine, then, gently though it flies from hill to hill, and sleeping lies on the valley and distant waters, is an agent of astonishing power, and of the most vital importance to all things around us. Though we cannot quite say, with the poet, that the glorious sun "plays the alchemist Turning, with splendour of his precious eye, yet, when we look at all the exquisite colours and forms which owe their existence to its beams, we can say that a ray of light fulfils a wonderful part in the chemistry of creation.Chemistry of Creation. CHAPTER LIII. THE OCEAN.-PART I. LET us transport ourselves to the seashore. What matter of thought and investigation lies before us in this restless and majestic element, whose waves cast themselves at our feet, shedding their salt spray over us! Are the principles of chemistry in movement throughout this mighty deep as they are on the earth and in the air, producing their slow but ceaseless changes of form and of matter? This inquiry, it is our present object to satisfy. It may, however, be premised that the chemistries of the Ocean are few and simple, far more so than those of earth or air; but they are not less interesting nor are they less important. We know but little of the real depth of our "sea of air," for it must be admitted, after all, that our data for the determination of this question are not wholly satisfactory; but we know, perhaps, less of the profundity of the great waters. All that can be learned is the depth down to which our researches have extended, in the vain search for the bottom. One of the greatest depths yet obtained was by Sir J. C. Ross, in S. Lat. 15° 3'. The weather being calm, and the water quite smooth, soundings were tried for with 4600 fathoms of line, or 27,000 feet. The depth of the Ocean is, however, by no means uniform, in consequence of the inequalities of the surface. at the bottom. Could we suppose the ocean emptied and the bottom exposed, we should behold a great cavity, very different from that of our imaginings. Far from its surface being smooth and uniform like the sides and bottom of some vast bowl-it would be seen that the many alternations of hill and dale, of mountain, rock, valley, and level plain, which give variety to the aspect of nature on land, are repeated in the bed of the Ocean; though, doubtless, a certain smoothness of aspect would be in general found to overspread these features, greater than we behold on land, in consequence of the levelling influence of currents, and of the deposit of sand and particles rubbed off rocks. Locke has well and simply said, that the sea is a collection of waters in the deep valleys of the earth. That such is really the case, is evident from the facts observed in sounding by means of the plumb-line. Shoals, for example, which extend for miles, and are surrounded on every side by deep water where the lead cannot find a bottom, are manifestly mountains in the Ocean, and would be seen as such were all the water removed. Sometimes the shore of a country falls with a very gentle inclination; sometimes, and particularly near precipitous coasts, the bottom within a few hundred yards of the rocks cannot be reached. The one would appear as a gently descending plain-the other as an abrupt precipitous mountain of great elevation. It must not, however, be supposed that no limit exists to the depth of the ocean, or that, except in the imagination of poets, it is without bottom. In all probability, its depth is only a fourth or a fifth part of that assigned to the air; the greatest hollows being supposed not to be deeper than from twelve to thirteen miles, or thereabouts. The pressure of the atmosphere is greatest on the surface of the earth. Not so with the Ocean. We must not forget, that on the surface of the earth we are at the bottom of the aerial sea; while, on the contrary, we are at the top, so to speak, of the sea of waters. Hence, as we descend into the Ocean, the superincum |