But heat exerts a repellent force among particles, and soon an
equilibrium is reached, for there comes a time when the contracting
body can contract no farther. But heat and light radiate away into
cold space, then contraction goes on evolving more light, and so
the suns flame on through the millions of years unquenched. It is
estimated that the contraction of our sun, from filling immensity
of space to its present size, could not afford heat enough to last
more than 18,000,000 years, and that its contraction from its present
density (that of a swamp) to such rock as that of which our earth
is composed, could supply heat enough for 17,000,000 years longer.
But the far-seeing mind of man knows a time must come when the
present force of attraction [Page 21] shall have produced all the
heat it can, and a new force of attraction must be added, or the sun
itself will become cold as a cinder, dead as a burned-out char.

Since light and heat are the product of such enormous cosmic forces,
they must partake of their nature, and be force. So they are. The
sun has long arms, and they are full of unconquerable strength
ninety-two millions, or any other number of millions, of miles
away. All this light and heat comes through space that is 200°
below zero, through utter darkness, and appears only on the earth.
So the gas is darkness in the underground pipes, but light at the
burner. So the electric power is unfelt by the cable in the bosom
of the deep, but is expressive of thought and feeling at the end.
Having found the cause of light, we will commence a study of its
qualities and powers.

Light is the astronomer's necessity. When the sublime word was
uttered, "Let there be light!" the study of astronomy was made
possible. Man can gather but little of it with his eye; so he takes