"are produced on plates of copper, coated over with silver, which are found to answer better than such as are entirely of the last-mentioned metal. After being washed with a solution of nitric acid, the plate is put into a well-closed box, where it is exposed to the action of iodine, a small quantity of the latter being placed at the bottom of the box, with a thin gauze between it and the plate. A layer of ioduret of silver is thus formed on the surface of the plate, and manifests itself by the yellow hue produced on the silver, which shows that the process of giving the plate the sensitive coating on which the action of the light delineates objects is completed. Thus prepared, the plate is placed within a camera obscura, of particular construction, and the delineation of the object is then effected in a very short space of time; but has to be afterwards brought out and rendered distinct by another operation, namely, submitting the plate to the action of vapour of mercury. Even then the process is not completed, for the plate has to be plunged into a solution of hyposulphate of soda, and afterwards washed in distilled water, which being done, the impression is fixed, and the plate may be exposed to light with perfect safety."[4] With reflection, refraction, and polarization of light, we have here no concern; therefore, interesting as the subjects are, we must pass them by.

[4] For further details, see the Handbook of Heliography, London, 1840; and (for preparing-paper) the Visitor, 1839, p. 290.

Light radiates from the sun with almost inconceivable velocity; that is, at the rate of nearly 200,000 miles in a second. Hence it is about eight minutes in traversing the intermediate space between our globe and its starting point or origin. But here comes a question not easily answerable. What is light? Is it matter? It is imponderable—can we conceive of matter without weight? Again, whence does it derive its velocity? The term radiation from the sun is convenient; but, then, what is radiation? Is there light, (and is there heat,) above the limits of our atmosphere? We cannot tell what the nature of the mysterious emanation of light and heat from the sun is, nor whether there are such phenomena as light and heat in empty space. Again, by what process is this enormous manufactory of solar emanation kept up? We may here lose ourselves in vague conjectures: He alone knows who said, "Let there be light: and there was light."

The few details which we have thrown together, bearing upon the natural light of our planet, derived from the sun, will suggest to the mind of the Christian those apt comparisons, of the broad light of day to the light of the gospel, transmitted to our souls from the Sun of righteousness. Till illuminated from above, man wanders in spiritual darkness; he sees neither the dangers that encompass him, nor the road by which he should go; he may have the light of nature and the light of science within him, but in spiritual affairs he is blind, and will be chosen only by the blind as a leader. Let, however, the light of revelation begin but to dawn upon him, and he learns to discover more and more clearly his real position, his true character, and the impossibility of his finding acceptance before God upon his own merits. He feels himself to be a sinful creature, and is led to rest upon the merits and atonement of the Son of God, who died for our sins, and rose again for our justification. Thus enlightened, he receives strength to pursue his journey to that bright and glorious kingdom of which God himself is the Sun, and where there shall no more be night, where "everlasting day abides," and all is glory and refulgence.

4. Its Temperature.—Heat and light are distinct from each other, though in many cases one accompanies the other, and is produced by the same cause, as, for instance, by combustion, electricity, percussion, the sudden condensation of air, etc. Heat without light, however, is developed by the physical or chemical changes of bodies, as by the condensation of steam, and the admixture of water with sulphuric acid. It results, moreover, from the vital and mysterious operations which are constantly taking place in organic beings, and especially in the classes of mammalia and birds. But the great source of heat—that upon which the temperature of our atmosphere mainly depends—is produced by the influence of the solar rays.

There is, indeed, heat in the body of the globe itself; to say nothing of volcanoes, we may state, that the deeper we penetrate into the earth, the higher does its temperature become—a circumstance which has led many philosophers to infer, that the centre of our globe is in a state of incandescence. It is not here that we ought to moot this theory, neither shall we enter into any speculations relative to the intrinsic nature of heat; like light, it is a mysterious agent or product, imponderable, yet subject to certain laws—laws which belong to a branch of philosophy into which it is not now our province to enter.

It may, however, be permitted us to state a few simple facts relative to the atmospheric temperature of our globe, and the adaptation of organic beings to the different degrees of heat within the inter-tropics, in the temperate and in the polar latitudes. Climate varies not only according to latitude, but also according to elevation, the relative proportions of land and water, the nature of the surface of the land, the extent of forests, etc. An island, for example, like England, surrounded by the sea, destitute of mountains of vast elevation, or of extensive morasses or forests, though lower in atmospheric temperature during the summer months than the parallel portions of the continent, has a milder atmospheric temperature during winter. The cold of North America during winter is greater than in the same parallels of latitude in the old world. Canada, for example, lies parallel to the northern half of Spain and France, (between 40° and 50° N. lat.) The severity of a Canadian winter is well known. At Quebec, the summer is that of Paris, the winter that of St. Petersburg. At New York, the summer is that of Rome, the winter that of Copenhagen. The same observations apply to the eastern portions of Asia. At Pekin, the scorching heat of summer is greater than at Cairo, while the winters are as rigorous as at Upsal.

To the different climates of the globe certain plants and animals are especially adapted; organic existence ranges from the poles to the equator. How varied, how multitudinous, how wonderful, are the forms and structures of organic creation, from the moss or lichen, which creeps upon the surface of the rock, to the towering palm or gigantic oak—from the microscopic animalcule or puny insect, to the ponderous elephant or enormous whale! But, as we have said, every distinct region has, in a general sense, its own flora and its own fauna. It is in tropical countries, beneath a fervid sun, that vegetation presents us with its utmost magnificence. There we see forests of evergreen trees, palms, and arborescent ferns—there bloom flowers of gorgeous hue, and luscious scent—there ripen fruits of most exquisite flavour, attractive both to the sight and the taste.

It is there, too, that the elephant, the hippopotamus, and the rhinoceros, the largest of terrestrial quadrupeds, roam the plain, or wander in the dense