And as the rays of heat and light accompany each other when emitted from luminous bodies, the velocity with which the rays of caloric move must be equal to that of light; and hence its particles must be equally minute. They differ however in this particular, that the rays of light produce the sensation of vision, and possess certain chemical properties, whilst in those of caloric the peculiar agency of heat resides.

LIGHT.

(SEE APPENDIX NO. 9.)

PART XI.

THE difference, in the day time, of what we perceive when our eyes are open and when our eyes are shut is produced by what is called light. The privation of light is called darkness.

It is this light which informs us of the presence of objects which are not near enough to touch our bodies, or which do not affect any of our other senses. Hence the blind must judge of the presence of the particular objects by the sound, or by the smell, or by the touch, &c. but not by means of light. In short, light does not sensibly affect any other part of our frame, besides the eyes.

The nature of light has occupied much of the attention of thinking men, and numerous opinions have been entertained concerning it. It has been sometimes considered as a distinct substance, at other times as a quality; sometimes as a cause, frequently as an effect: by some it has been considered as a compound, by others as a simple substance. Philosophers of the present day are not agreed as to the independent existence of light, or the cause by which we see.

This is not the place for discussing the respective merits of all the ingenious hypotheses which have been advanced concerning this subject: we shall assume the doctrine of those who admit the independent existence of light, not only because it is more generally received, but is also best calculated to explain its chemical properties.

SECT. I.

NATURE OF LIGHT IN GENERAL.

of

LIGHT is that which proceeds from any body producing the sensation of vision, or perception of other bodies, by depicting an image of external objects, on the retina of the eye. Hence it announces to animals the presence the bodies which surround them, and enables them to distinguish these bodies into transparent, opaque, and coloured. These properties are so essentially connected with the presence of light, that bodies lose them in the dark, and become undistinguishable.

Light is regarded by philosophers as a substance consisting of a vast number of exceedingly small particles, which are actually projected from luminous bodies, and which probably never return again to the body from which they were remitted.

They have a determinate size; they are at a certain distance from each other; they move with a certain velocity, and have a certain momentum. They are universally expanded through space; they exert peculiar actions; and are obedient to the laws of attraction, and other properties of matter.

Several remarkable discoveries made in astronomy, and in other branches of natural philosophy, give us some ideas concerning the size, distance, velocity, &c. of the particles of light.

1. EXPLANATION OF CERTAIN TERMS OF LIGHT.

In order to facilitate the doctrine of light, we shall shortly explain a few terms made use of by philosophers when treating of it; namely:

A ray of light is an indefinitely small portion of light as it comes from a luminous body.

A medium is a body which affords a passage for the of light.

rays

A beam of light is a body of parallel rays.

A pencil of light, or a pencil of rays of light, is a body of diverging or converging rays.

Converging rays are rays which tend to a common point. Diverging rays are those which come from a point, and continually separate as they proceed.

The rays of light are parallel, when the lines which they describe are so.

The radiant point is the point from which diverging rays proceed.

The focus is the point to which the converging rays are

directed.

2. SOURCES OF LIGHT.

Light is emitted from the sun, the fixed stars, and other luminous bodies. It is produced by percussion, during electrization, combustion, and in various other chemical processes.

Why the sun and stars are constantly emitting light, is a question which probably will for ever baffle human understanding; at any rate, it is not connected with chemistry.

The light emitted during combustion, or chemical action, exists previously, either combined with the combustible body or with the substance which supports the combustion. The light emitted during percussion and electrization, &c. is foreign to our subject.

SECT. II.

PHYSICAL PROPERTIES OF LIGHT.

1. VELOCITY OF LIGHT.

The velocity of light is much greater than that of sound; for the flash of a gun fired at a considerable distance is seen before the report is heard.

The clap of thunder is not heard till some time after the lightning has been seen, except the phenomena takes place at no great distance.

The motion of light is so very rapid, that in all common experiments it appears to be instantaneous. Its velocity cannot be measured between the tops of the most distant mountains that are visible from each other. Astronomers have, nevertheless, found that it may be measured when it moves between some of the heavenly bodies. For by the eclipses of Jupiter's satellites, it appears that it takes 8 minutes and 13 seconds in passing across the semidiameter of the earth's orbit. From these accurate investigations they have deduced that light moves at the astonishing rate of about 167,000 geographical miles in one second, or that it passes from the sun to the earth in little more than eight minutes; and nevertheless, according to geometricians, light, from its departure from the nearest of the fixed stars, which is, in truth, at least four hundred thousand times more remote than the sun, is near six years in its progress to our eyes; so that a star placed at that distance would be still visible for six years after its destruction, supposing that process to take place. How grand and sublime are the meditations suggested by this object on the immensity of space, the boundless extent of the universe, the vast number of globes which traverse through it, and the duration of time which they measure in their silent march!

The velocity of light was first noticed by Roemer, and afterwards determined and established by Dr. Bradley's ingenious theory on the aberration of the lights of the fixed stars. The principles on which this discovery is founded may be familiarly explained as follows:

Suppose a tube to be erected perpendicular to the horizon at a time when it rains, the drops falling perpendicularly down; and suppose the diameter of the tube to be such as to admit but one drop at a time; then it is plain, that if a drop of water enter the orifice of the tube it will fall to the bottom without touching its sides. But if the tube, without altering its perpendicularity, be moved along in the direction of the horizon, any drop that enters will strike against one of its sides, and none will pass clearly through while the motion continues, unless the upper end of the tube be also inclined towards the part to which its motion is directed.