the individual may be made to understand that he wants some faculty which those around him possess, there cannot be however any consciousness of privation where there never had been actually any enjoyment; or where there was no recollection of it, if it had for a time existed. And even in the case of individuals who have been deprived of sight long subsequently to birth, although the recollection of the former enjoyment must more or less imbitter their present state; yet so long as the offices of surrounding friends are the means of administering to their comfort, more especially if those offices are fulfilled with kindness, the mind soon becomes reconciled to the privation for it is a fact, repeatedly observed, that blind persons under such circumstances are usually cheerful. Nor ought we to forget the compensation which nature affords to those who are deprived of sight, in the consequently quickened activity of some of the other senses.

Let us however suppose for a moment that, all the faculties and recollections of man remaining unaltered, and the general processes of nature continuing, if possible, the same as they are now, the existence of light were withdrawn from this earth: what would then be the condition of mankind? How could those occupations of life be pursued which are necessary for the supply of our simplest wants? Who in that case should yoke the ox to the plough, or sow the seed, or reap the harvest? but indeed under such a supposition there would soon be neither seed for the ground, nor grain for food: for, if deprived of light, the character of vegetation is completely altered; and its results, as far as general utility is concerned, destroyed. Or suppose, further, that these necessary supplies of life were no longer required, on account of some consequent alteration in our physical constitution; or that they were procured for us by any unknown means; yet, in all the higher enjoyments of our nature, how cheerless, how utterly miserable

would be our situation. Under such circumstances, wisdom would not only be

"at one entrance quite shut out,"

but no other entrance could then be found for it; for of the other senses, the only remaining inlets of knowledge with reference to an external world, there is not one, which, if unaided by sight, could be of any practical value. With respect indeed to our inward feelings, though we should, on the one hand, be spared, by the privation of light, the worse than corporeal pain of the averted eye of those who ought to meet us with gratitude and affection; we should, on the other hand, lose the beams of filial or parental love; of which even a momentary smile outweighs an age of pain.

As in mathematical reasoning the truth of a proposition is sometimes indirectly proved by showing that every process of proof but the one proposed would lead to an absurd conclusion: so, though the supposition of a general and total privation of light is on all probable grounds of reasoning inadmissible, it may yet serve to show us indirectly the value of the good we enjoy. But it is sufficient to have given a few instances of the necessary effects of such a privation and it will be a more grateful task to enumerate the actual benefits which we derive from the agency of light.

In the vegetable world, upon the products of which animal existence ultimately depends, light is the prime mover of every change that takes place, from the moment the germ emerges from the soil. Exclude the agency of light, and in a short time the most experienced botanist might possibly be at a loss to know the plant with which he is otherwise most familiar; so completely obliterated are all its natural characters, whether of colour, form, taste, or odour. Thus the faded colour of the interior leaves of the lettuce and other culinary vegetables is the result of such a degree of compression of the body of the

plant as excludes the admission of light beyond the exterior leaves. And, again, if a branch of ivy or of any spreading plant happen to penetrate during the progress of its vegetation into a dark cellar, or any similar subterraneous situation, it is observable, that, with the total loss of colour, its growth advances with great rapidity, but its proportions alter to such a degree as often to mask its original form. And, lastly, which in a practical point of view is of the greatest importance, if a plant which has grown without the influence of light be chemically examined, its juices, it might almost be said its whole substance, would be found to consist of little else than mere water; and, whatever odour it may have, is characteristic, not of its original nature, but of its unnatural mode of growth; becoming, in short, very like that of a common fungus. The total result is, that all the native beauties and uses of a vegetable growing under these circumstances are lost: the eye is neither delighted by any variety or brightness of colour; nor is the sense of smell gratified by any fragrance: the degeneracy of its fibre into a mere pulp renders it unfit for any mechanical purpose; and the resinous and other principles on which its nutritive and medicinal virtues depend, cease to be developed. In some instances, however, the bleaching or etiolation of plants is useful in correcting the acrid taste which belongs to them in their natural state; as in the case of endive and of celery.

Its

The effect of light upon vegetation has been selected in the preceding paragraph as affording the most powerful instance of the adaptation of this natural agent to the physical condition of man. effects upon individuals of the mineral and animal kingdom are neither so easily to be traced, nor are nearly so important in their consequences, at least in a practical point of view; and therefore it is not proposed to bring them forward in a more particular

manner.

The observation of those modifications which light

undergoes when reflected from the surfaces of bodies has given rise to one of those impressive arts which are capable of contributing no less to the refinement of society at large, than to the gratification of the individuals who cultivate or admire them. For who can look on the productions of such masters as Guido, Raphael, or Michael Angelo, without imbibing a portion of the spirit which animated those masters in the execution of their inimitable works? or, if we quit the regions of imagination and of history, and descend from the higher efforts of the art into the retirement of domestic life, who can successfully describe those emotions which are excited by the portrait of a beloved object, a child or parent now no more; or by the representation of that home and its surrounding scenery, in which the careless and happy hours of childhood were passed?.

The intrinsic source of the pleasure which we experience from the contemplation of a painting is probably to be sought for in that principle of our nature, of more extensive influence perhaps than is generally supposed, which derives a gratification from perceiving the resemblance of actual or probable truth; or even, and sometimes in a higher degree, from the delineation of fictitious characters and scenes and hence the art of painting is easily made the vehicle of the ludicrous and the horrible, no less than of the sublime and the beautiful: and, hence also, the painter may incur a considerable degree of moral responsibility in the exercise of his art. But this view of the subject, though fertile in reflections of great moment, and practically too much neglected, does not belong to the purpose of the present treatise.

SECTION III.

Heat.

FROM the consideration of the subject of light, the mind passes by a natural transition to that of heat:

for these agents, though not necessarily or always, are in reality very often associated together: and they are each of them characterised by the want of that property which almost seems essential to matter, namely weight. In their relation to the physical existence of man and animal life in general, there is this difference between them-the presence of light is only indirectly necessary; the presence of heat is directly necessary. Different degrees of heat indeed are requisite for different species of animals: but if the heat to which any individual animal be exposed be much below that which is natural to the species, and be continued for a sufficient length of time, all the vital functions are eventually destroyed; or, as in the case of the hibernation of particular species of animals, are at least partially suspended.

The degree of heat adapted to the human frame is so nicely adjusted to the bodily feelings of man, that, if we take a range of fifty degrees of Fahrenheit's thermometer as indicating the average extent of variation to which the body is exposed in this climate, it will be found that a difference of two or three degrees, above or below a given point, will generally be sufficient to create an uncomfortable sensation. The late Mr. Walker, whose experiments on the artificial production of cold are well known to the philosophical world, ascertained that the point of 62° or 63° of Fahrenheit is that, which, upon an average of many individuals, is in this climate the most congenial, as far as sensation is concerned, to the human body. But it is a merciful provision of nature, considering the numerous vicissitudes of human life, that man is capable of resisting very great and even sudden alterations of temperature without any serious inconvenience. Thus an atmosphere so cold, as to depress the mercury in Fahrenheit's thermometer to the 52d degree below the freezing point of water, has been borne under the protection of very moderate clothing. And, on the other hand, an atmosphere of a temperature as high as the 200dth degree of