this difference arises from their different magnitudes, seems evident from the different directions in which the several sorts of rays move, after they have passed through a budy of glass, water, &c. of some special figure, especially that of a prism.

Since the weight of bodies is proportional to the quantity of matter, it follows, that, where the latter is diminished indefinitely, the former will be so too; therefore the weight of light must be imperceptible. Boerhaave caused a globe of iron, twelve inches in diameter, to be heated red-hot, and suspended at the end of a very exact balance, and nicely counterpoised by weights at the other end, and thus let it bang till all the particles of heat or light were escaped, when he found the equilibre of the balance in no wise altered.

That the particles of light have not only magnitude, but, also, in different degrees, is another, and perhaps the most subtil, discovery of the Newtonian philosophy. This is absolutely proved by the different refrangibility they are found to display in passing through a prismatic figure of glass or water; for the power of the prism detains the issu ing particle, and draws it a little towards the surface; and, since this power is the same, it would bave the same effect on all the particles of light, if they were all of an equal magnitude, because they have all an equal velocity. But since this effect is different

among the particles, some being detained and drawn aside to a greater distance than others, it follows, they must be less in magnitude, to become more subject to the influence of the attracting surface; in like manner as the electric effluvia will act upon and agitate very small and light bodies much sooner, and more easily, than they can move those which are larger.

According to some very elaborate experiments of Dr. Bradley, light moves at the rate of one hop. dred and vinety-tive thousand two hundred and eighteen miles in a second. Another account states the number of miles at one hundred and seventy thousand. The velocity of light exceeds that of a cannon-ball by one million five hundred and fifty thousand times. It is calculated to travel fromai the sun to the earth in eight minutes and thirteen seconds.

II. “ Of light, in chemistry.” Light is consi-dered, by modern chemists, and apparently with reason, as a simple elementary body; but they have not yet been able to form a theory on this subject, in which considerable difficulties are not involved.

Light manifests itself to the mind through the mediums of two senses. To the organs of vision it presents forms and colours; and to those of touch, the phenomenon of heat. It is observable, that experience so uniformly teaches us to unite the ideas of light and heat, that none but the philosopher, and he scarcely with intelligibility, would talk of fire that is not luminous, or light that is not warm : the first step, therefore, toward an analytical examination of this matter, is to separate in our minds, the warmth and the light of our hearths. To assist this attempt, we must remember that fire certainly can exist, without the company of light, since both coals and iron may frequently be met with in a state of perfect blackness, combined with violent heat,

To instance the existence of light without warmth, as in the temperature and brightness of a frosty day, is equally easy; yet, after all, light and heat still recur to the mind as inseparable things; and even of those habituated to abstract considerations, numbers are disposed to affirm that they are one and the same.

That they are not one and the same, has certainly pever been proved; but this, of the two, appears the more probable conclusion. Heat, as is shown in the article Fire, is one of the attributes of caloric; but caloric is not conceived to possess the attribute of light, also; and though heat and light are usually found together, this connection has been attributed rather to mutual antipathy than to homogenuousness.

The question is, Why do combustible bodies, in the act of consuming, give out ligbt? It is answer. ed, Because there is a repulsion between light and caloric.

Taking for granted the previous proposition, that all combustible bodies, in greater and less degrees, .contain both caloric and light, it is agreed that, supposing the reality of the repulsion, these two substances cannot be accumulated in the same body beyond a certaiu degree. Arrived at that, if the. caloric predominate, it will tend to drive off the light; if the light, on the contrary, happen to prevail, it will displace the caloric. It is on this prin!" ciple that light flies off, in the form of fame, during combustion

The present state of this inquiry will not allow another conclusion than an abrupt one, to this arzicle. The properties of light, and the extent of

the subject, have been indicated; but to fix the ideas of the reader, with respect to the explanation, would, probably, be only to fix them in error.

LIGHTS, northern. See AURORA.

LIGHT HORSE, in military economy, English troops, mounted on swifter horses, and more lightly accoutred, than the life-guards, or heavy horse.

LIGHTER, in naval architecture, a large kind of boat, used in the river Thames for carrying heavy goods, as coals and timber.

LIGHTNING, in meteorology, a flash of light snddenly appearing in the atmosphere, and commonly disappearing in the same instant; sometimes attended with clouds and thunder, and sometimes not.

Lightning is proved, by the experiments of Franklin, to be produced by the electric fluid. *Thunder is the explosion of clouds charged with - that fluid. Lightning is to thunder, what the flash is to the report of gunpowder.

A very remarkable property of lightning, the zigzag kind especially, when near, is its seeming omnipresence. If, when a clap of thunder, accompanied with this species of lightning, 'occurs, two persons are looking different ways, both will per. ceive the flash; not only that indistinct illumina

tion of the atmosphere which is occasioned by fire 2. of any kind, but the form of the lightning itself;

and every angle it makes in its course, will be as distinctly seen by each, as if they had looked directly at the cloud whence it proceeded: and if a person were at that moment looking at a book, or any other object, that he held in his hand, he, also,

3

would distinctly see the form of the lightning, between his eyes and the objects. This property seems peculiar to lightning.

The different forms of the flashes of lightning are all equally found in electric sparks; so that an account of the origin of this difference of form may, by analogy, be drawn. Where the quantity of electricity is small, and, consequently, incapable of striking at any considerable distance, the spark appears straight, without any curvature, or angalar appearance; but where the electricity is very strong, and, of consequence, capable of striking an object at a pretty considerable distance, it assumes a crooked or zig-zag form.

LIGNUM vita. The lignum vitæ tree is a native of the West Indies, and the warmer parts of America : there is also a species, a native of the Cape of Good Hope. It is a large tree, rising at its ful growth to the height of forty feet, and measuring from fifteen to eighteen inches in diameter; having a hard, brittle, brownish bark, not very thick. The wood is firm, solid, ponderous, very resinous, of blackish yellow colour, in the middle, and a hot aromatic taste. It is so hard as to break the tools which are employed in felling it; and is, therefore, seldom used as firewood, but is of great use to the sugar-planters for making wheels and engs to the sugar-mills. It is also frequently wrought in bowls, mortars, and other utensils. It is imported into England, in large pieces of four or five hundred weight each, and from its hardness and beauty, is in great demand for various articles in the turuery ware, and for trucks of ship blocks. The wood, gum, bark, fruit, and even the flowers

a

VOL. III,