Light, signifies that invisible fluid which renders objects perceptible to the sight.- The velocity of light is almost inconceivable, though its motion is not instantaneous : the particles of light fly nearly 200,000 miles every second of time, which is above a million of times swifter than the passage of a cannon-ball. And, as half the diameter of the globe we inhabit, extends to 90,000,000 of miles, the rays of the sun travel that vast distance in 8 1/4 minutes, before they illumine our earth.
The nature of light has, at all times, been a subject of speculation among philosophers ; and various theories Lave been formed and rejected, or succeeded by others that were more plausible, though inconclusive. Sir Isaac Newton conjectured it to consist of rays, some of which possessed a greater degree of refrangibility than others: subsequent philosophers are not yet agreed, whether light and heat are the same fluid under different modifications, or two distinct fluids which are frequently united. Amid this concussion of opinions, it is difficult to select the most probable ; we shall therefore briefly state the effects of this agent upon the vegetable and animal creation.
Light is essentially necessary to vegetation : in our climate, it is seldom productive of injury by its excess, though the want of it is often pernicious. This fluid acts as; a kind of stimulus on the more irritable parts of plants; as a pears from the sun-flower and others, that expand or bow when exposed to the rays of the sun : hence Dr. Darwin supposes there may be diseases in plants, arising from the excess of this stimulus, but which, he thinks, have hitherto-been disregarded. To corroborate this conjecture, he specifies the Goat's-beard, or Salsafie (tragopo-gon, L.), and some other vegetable productions, which close their flowers about noon, in order to prevent the influence of such stimulus.
On the contrary, the absence or defect of light produces a disease called etiolation, or blanching, because vegetables deprived of that fluid become white. It has a similar effect; on animal bodies ; which, as Dr. Darwin has pointedly remarked, are in consequence rendered pale and inert: this is confirmed by the languid and etiolated countenances of the young ladies in some boarding-schools, where, from false motives of delicacy, they are secluded both from the light of the sun, and the invigorating influence of the air : it is still farther evinced in those studious persons, who pass their waking hours in un-ventilated apartments, especially during a considerable part of the night.
But, though such etiolation be naturally injurious to vegetation, it has been artificially employed with success, in rendering certain plants esculent, by depriving them of their acrimony, cohesion, and colour. This method is chiefly practised on celery, by earthing up that plant nearly to the top; on sea-kale, by covering it entirely with horse-litter or straw; and, lastly, on lettuces and endive, by tying the root-leaves together with a band2' bandage.—On the chemical effects of light, we are silent; for the opinions respecting them are not less unsettled than those concerning its nature.- 'The inquisitive reader will find ample information on this subject, in the writings of Dr. Priestley, and also in the different volumes of the Transactions of the Royal Society.