3. _Its Illumination._--Light is evolved by and during the combustion of various bodies, as wood, coal, tallow, and spirits, or alcoholic fluids; it is the result of electricity--that is, of the electric spark or flash; it is evolved during many chemical experiments, and by the attrition of two quartz pebbles. The percussion of flint and steel produces luminous sparks; these sparks consisting, as the experiments of Mr. R. Hook have proved, of minute ignited globules of iron, struck off at the moment of percussion. These, and many other sources whence light proceeds, need not here be detailed; our object is with that general light, the light of day, which results from the influence of the solar rays. For the reception of pictures on the retina by means of the rays of light reflected from all things within the scope of vision, the eyes of animals are especially constructed. To some animals a dim twilight suffices, so far as regards their personal maintenance and their enjoyment of life; to a few the little light, which even in the darkest night is not absent, is all that is required for their ordained habits and economy. In such instances, the retina is exquisitely susceptible, and the iris dilates to admit the entrance of every feeble ray reflected from the objects either of their search or avoidance. Often have we seen the owl, when twilight has melted into darkness, skimming over the fields, along the lane, and around the barn, quartering the ground in search of mice or moles, which, had they been at our feet, would to our eyes have been invisible. On the other hand, they would have been nearly, if not quite invisible to the owl in the full glare of sunshine. Yet, be it understood, light is as essential to the night-bird as to men, or the giraffe of the glowing wilds of Africa. We know not what _total darkness_ is, unless perhaps we be walled up in a deep dungeon underground, to which light is utterly denied access; such living tombs have been contrived by the diabolical minds of tyrants, pagan and papistical. In the darkest night, even our eyes dimly discern the "form of things obscure"--things which, to a nocturnal animal, would be boldly conspicuous. Light is a stimulus both to the animal and vegetable worlds. It is when daylight breaks that "man goeth forth unto his work and to his labour until the evening;" it is when light breaks that the feathered songsters of the grove join in one chorus of--why may we not say so?--instinct-urged thanksgiving. O man, gifted with reason! O man, the heir of immortality! utter, then, thy song of praise, of gratitude, that the light of another day is bestowed upon thee: and then be diligent in all that God has called upon thee to perform, seeing that "the night cometh, when no man can work."