what hap

Immediately day-light is withdrawn, this At night power ceases; hence vegetables grown in pens. cellars are blanched and watery. They want carbon to give them their healthy green tint, and to make them stout and firm.

of its use.

Indeed so useful, and probably essential, is A proof light to the well-being of the animal and vegetable world, that before either were created, the Great Architect provided light.

SECTION III.

THE SUN AS A SOURCE OF LIGHT.

made when.

That light existed before the creation of the The sun sun we are quite certain from the plain statement of the Bible. On the first day of creation, if not before, God said, "Let there be light;" but did not create the sun and moon till the fourth day.

the Bible.

The words of scripture upon this subject are What says well-worthy of notice, "God said, let there be lights in the firmament of heaven, to divide the day from the night." That was their first great object.

their next.

Their next was this: "And let them be for What was signs, and for seasons, and for days, and years."

teaches us.

Here the objects of the sun and moon are What this distinctly stated to be the following: to separate the day from the night, and to subdivide time into days, seasons, and years.

not said.

It is nowhere said that light is dependant What is on the sun. It existed before the sun was created; and if "God is light," it exists where the sun has no influence.

is its use.

All that the sun does in the matter, is to what then call light into action in our system; and if it

What once

be true, that light is the effect of vibrations in the luminous ether, the great function of the sun is simply to set this ether into motion.

It was long thought that the sun was a was thght. luminous body like a red-hot coal; and that the light of day was due to the sun in much the same manner as the light of a room may be due to a lamp.

What now

It has, however, been satisfactorily proved is known. that the sun is not luminous, but perfectly opaque, like the earth we live on; and that heat and light do not emanate from the sun, but from a luminous atmosphere which invests it. The sun, in fact, seems to be surrounded with two atmospheres, an inner and an outer.

What is the fact.

How differ they.

How far is the sun off.

What are

sun rays,

What their

functions.

The inner atmosphere, like our own, is not luminous; the outer one, called for the sake of distinction, its photosphere, is of incandescent gas. It is this latter which produces the vibrations of ether, and is the cause of light.

The distance of the sun from our earth is 95-millions of miles; the vibrations of light travel nearly 12-millions of miles in a minute, and therefore a ray of light is somewhat less than eight minutes coming from the sun to our earth.

Sun-rays are of three sorts, calorific, luminous, and chemical. To these a fourth class is sometimes added, termed magnetic. All these rays collected into one bundle constitute a beam.

Calorific rays produce heat, but not vision and colour. Luminous rays produce vision and colour, but not heat. Chemical rays produce neither heat, vision, nor colour, but certain

chemical effects on the composition of bodies; and magnetic rays produce magnetism.

ours what.

Calorific rays are red; luminous rays, yellow; Their colchemical rays, violet; and magnetic rays seem to be associated with these last. Phosphorus is kindled by the red rays, and extinguished by the violet.

tographic.

For photographic purposes the chemical or Which phoviolet rays work with the best effects; and hence the focus for photographic impressions is somewhat nearer the lens than the true visible focus.

Which the

The calorific rays are the least refrangible, strongest. the luminous more so, but the chemical the most. Hence when a sun-beam is divided by a prism, the blue is bent to the top, the yellow occupies the middle, and the violet comes at the bottom.

bows how.

So in a rainbow, the red occupies the outer In rainor larger arch, the yellow the middle band, and the violet the innermost or smallest curve.

SUN-RISE. Suppose the earth to turn from west to east, it is evident that the ray AR will reach the horizon before either AY or AB.

SUN-SET. It is equally evident, if we suppose the figure reversed, that the ray AR will remain upon the horizon, after AŸ and AB are hidden from sight by the curve of the earth.

N.B. AR represents red rays, AY the yellow, and AB the blue.

What else follows.

How at

sun-rise.

It is in consequence of the smaller refrangibility of the red rays that they prevail at sun-set, when the blue especially are so bent that they are out of sight some time before the red are intercepted by the curve of the earth.

At sun-rise the same phenomenon occurs, because the red rays reach the horizon before the blue, which are more refracted or bent.

An illus

tration.

Apply this to sound.

Apply this to light.

not occur.

SECTION IV.

ANALOGY BETWEEN LIGHT AND SOUND.

§ 1.-NATURE OF SOUND AND COLOUR-WAVES.

When a stone is thrown into a pond, ripples of water are propagated round the point of disturbance, and circulate in concentric circles. So when a sounding body vibrates, the vibrations impinging against the air, produce somewhat similar ripples in that fluid; and these ripples circulate round the sounding body, in much the same manner.

When again, any body like the sun, a fire, or a lamp is luminous, or when any surface, such as that of a house, a flower or a bird is visible, it vibrates like a bell or harp-string; and these vibrations being communicated to the ether, create therein concentric waves.

What does It must not be supposed in any of these instances that the water, air, or ether, moves from wave to wave. This is not the case at all; the ripple moves, the form of the wave moves, but the aura in each case is stationary.

*

* Aura is a word used to express the substance of magnetism, odour, electricity, and other currents too subtile to be called fluids.

corn-field: Illustrate.

This may be illustrated by When the wind blows, the standing corn is thrown into wavy motion. The waves seem to run from one end of the field to the other; but we all know that the corn remains rooted to the same spot. The motion is not that of the corn, but that of the ripple or the wave.

§ 2.-RELATIVE VELOCITY OF SOUND AND
COLOUR WAVES.

the pitch.

The pitch of any sound depends on the What gives rapidity with which sound-waves succeed each other; the more rapid the succession the higher the pitch, and the less rapid this succession the lower the pitch.

Thus in the "pitch A" of an orchestra the Exemplify. pulsations of the air follow each other at the rate of 853 in a second. In the lowest C of a seven-octave piano, they follow at the rate of 32 in a second. But in the highest C of the same instrument, they are at the rate of 7680 in a second.

note what.

The deepest or gravest sound that the ear The lowest can appreciate is one that proceeds from only two vibrations in a second. This note is four octaves lower than the lowest C of a sevenoctave piano.

highest.

The highest or shrillest sound which the What the human ear can transmit to the brain corresponds to 73,000 vibrations in a second. Somewhat more than four octaves higher than the highest C of the same instrument.

hearing.

The perception of sound is produced by the What is drum of the ear vibrating in sympathy with

S