is continually rising from the surface of the earth, as well as of the ocean and every lake and river. But, in addition to this aqueous vapour, the air is also charged to a variable extent with light and heat and electricity of which the two first are so obviously adapted to the wants of man as to demand immediate attention. Electricity is probably of equal importance in its relation to man: but the true character of that relation has not yet been sufficiently developed to call for a distinct consideration on the present occasion.

SECTION II.
Light.

THE metaphorical expressions of all ages and nations, with respect to light, sufficiently evince the value in which that inestimable gift is held. In the sacred Scriptures indeed, not only are temporal blessings compared to light, and temporal evils to darkness; but holy deeds are frequently described under the character of the former; and unholy deeds under the character of the latter: and, with respect either to classical or oriental literature, a thousand instances might easily be adduced illustrative of the same metaphorical use of the terms in question.

When, after a dark and tempestuous night, the mariner first perceives the dawn of returning day; although that dawn discover to his view the evil plight to which the storm has reduced his vessel, why does he still hail day's harbinger as his greatest relief, but because without the aid of light he could not possibly extricate himself from the difficulties of his situation? Or, when the child, awakened from its sleep, finds itself alone in darkness, why is it overwhelmed with terror, and why does it call out for protection, but from the influence of those undefined fears, which naturally occur to the mind under the privation of light?

There is something so congenial to our nature in light, something so repulsive in darkness, that, probably on this ground alone, the very aspect of inanimate things is instinctively either grateful or the reverse, in consequence of our being reminded by that aspect of the one or of the other: so that on this principle, perhaps, particular colours throughout every province of nature are more or less acceptable in proportion as they approach nearest or recede farthest from the character of light, whether reflected immediately from the heavenly bodies, or from the azure of the sky, or from the thousand brilliant hues with which the setting or the rising sun illuminates its attendant clouds.

In illustration of the principle just advanced, gold and silver among metals might be opposed to lead and iron and, among flowers, the brilliancy of the crocus, the lily, or the rose, to the lurid aspect of henbane or belladonna. And though something of a moral character may in these instances determine the preference; yet there is nothing unreasonable in supposing, that, as the instincts of the inferior animals regulate their tastes and distates to natural objects; so there may also be in the case of human beings congruities, or the reverse, between the sense impressed and the object impressing it. In fact, with respect to that sense, the organ of which is the ear, it is known that infants shrink back from deep sounds, and express delight at acute sounds, long before any intellectual or moral feeling can sway them; and, correspondently with this assertion, the lullaby of the nurse partakes, among all nations, of the same essential character. It is a fact equally deducible from observation, that particular flavours and odours are naturally acceptable, or the reverse, to children. And again, with reference to the sense of touch, smooth surfaces almost universally give a pleasing impression; which is not imparted by rugged surfaces. Why then may it not be the same with respect

to the sense of sight, in the case either of colour or of form?

The abundant supply of light from its natural source the sun, and the ease with which it is producible by artificial means during the absence of that luminary render us habitually less sensible of its real value, than undoubtedly we should be, were we to experience a long continued privation of it. And as to the regularly periodical privation of it which we experience in consequence of the alternation of night with day, this is so far from being an evil, that it is obviously beneficial; inasmuch as, in consequence of this very absence, sleep is both directly and indirectly conciliated: without which gift of Heaven, all our faculties would soon be exhausted, and all our happiness consequently extinguished.

The beneficial influence of sleep on our whole frame is too obvious in its effects to require any formal demonstration: but it will be interesting to consider its relation to the absence of light. It appears then that, by a fundamental law of our nature, a sense of uneasiness invariably follows a long continued exercise of our powers, either corporeal or mental: and, unless this sense of uneasiness have been produced by too inordinate exercise, it is soon relieved by that state of the system which we call sleep; during the continuance of which, provided it be sound and of a perfectly healthy character, all the voluntary muscles of the body become relaxed, and the nervous system remains comparatively inactive; the whole body acquiring by this temporary cessation of its energies a renovated accumulation of those powers, which are necessary for the purposes of active and intellectual life.

In order to dispose us to yield to the sensation of approaching sleep, the periodical succession of night to day has been ordained by nature. For, with the approach of darkness cease all the usual stimuli of that sense, which is accommodated to the impulse of light, and which calls our faculties into action more

G

frequently than any other; nor is the intention of nature less evident, because, either from avarice or the dissipation of luxury, some individuals protract the labours or the pleasures of the day beyond the natural period assigned for those purposes; since these are unnatural exceptions to the observance of the general law.

Although it would be difficult to prove directly that there is any necessary connexion between darkness and sleep, yet this connexion is rendered at least highly probable by the effect usually produced on the approach of darkness upon animals in general, but more remarkably on birds; for, with the exception of those whose habits are nocturnal, all birds betake themselves to sleep as soon as night approaches: and if darkness should anticipate night by many hours, as happens when any considerable eclipse of the sun takes place in the middle of the day, we still find the birds of the field as well as our domesticated fowls give the same indications of composing themselves to sleep, as at the regular period of sunset. If it should be said that this does not more serve to prove a connexion between darkness and sleep with reference to these animals, than to prove the effect of a long continued association resulting from their habit of going to roost at sunset; it may be asked, why should darkness, unless from some inherent cause, lead them to compose themselves to sleep at the hour of noon, instead of the usual hour of evening; since, on the one hand, periodical states of the animal system do not usually recur before the termination of the habitual period; and, on the other hand, the individuals cannot at so early an hour have experienced such a degree of exhaustion as would of itself invite to sleep?

In stating that the voluntary action of the muscles ceases during sound sleep, we ought not to omit the remarkable fact that those muscles which are not under the empire of the will continue their action uninterruptedly through the deepest sleep. Of all

the muscles of involuntary motion, this observation holds most remarkably with respect to the heart; the continued action of which organ during sleep is a phenomenon worthy of the deepest attention of a philosophical mind. All other organs of the body have their periods either of absolute or comparative rest; the senses are in a measure periodically locked. up by sleep during one quarter at least, if not one third of our whole existence; the limbs of the most athletic individual lose their power of motion after a few hours of unremitted exertion; even the brain, which during the hours of sleep and the interruption of all the common functions of the body frequently represents to the internal senses the most busy scenes of active life-even the brain may be exhausted by unusual fatigue, or other causes, and may thus involve the general system in the stupor of apparent death-but the heart, unless on such occasions as the momentary interruption of a swoon, never rests: so that, whether we look back to that period of our existence, when, in our yet imperfect state, there could scarcely be discovered the faint outline of those members, which in after life constitute man's strength and beauty, the presence of the heart may be recognised by the impulse of its vibratory motion, though its form is yet undefined, or at least indistinguishable; or whether, on the other hand, we look forward to the latest moments of protracted disease, or expiring old age, the same organ is the last part of our frame which continues to give immediate proof of vital

motion.

The privation of light is rarely, if ever, total; for though the empire of time is divided in nearly equal proportion between day and night, there are comparatively few nights in which there is not diffused through the air a sufficient quantity of light for many of the purposes of life. Nor, with respect to those persons who either were born blind, or became blind in early infancy, is the absence of light felt with any degree of severity; for, in such instances, although