ation of light or heat is produced.

We see the water waves, but we can never see with the eye the heat and
light waves which roll in to us from that far-distant source, the sun.
We can be sure of them only through their effect on our bodies, and by
the visible work they do.

140. How Heat and Light Differ. If heat and light are alike due to
the regular, rapid motion of the particles of a body, and are
similarly conveyed by waves, how is it, then, that heat and light are
apparently so different?

Light and heat differ as much as the short, choppy waves of the ocean
and the slow, long swell of the ocean, but not more so. The sailor
handles his boat in one way in a choppy sea and in a different way in
a rolling sea, for he knows that these two kinds of waves act
dissimilarly. The long, slow swell of the ocean would correspond with
the longer, slower waves which travel out from the sun, and which on
reaching us are interpreted as heat. The shorter, more frequent waves
of the ocean would typify the short, rapid waves which leave the sun,
and which on reaching us are interpreted as light.

CHAPTER XV

ARTIFICIAL LIGHTING

141. We seldom consider what life would be without our wonderful
methods of illumination which turn night into day, and prolong the
hours of work and pleasure. Yet it was not until the nineteenth
century that the marvelous change was made from the short-lived candle
to the more enduring oil lamp. Before the coming of the lamp, even in
large cities like Paris, the only artificial light to guide the
belated traveler at night was the candle required to be kept burning
in an occasional window.

With the invention of the kerosene lamp came more efficient lighting
of home and street, and with the advent of gas and electricity came a
light so effective that the hours of business, manufacture, and
pleasure could be extended far beyond the setting of the sun.

The production of light by candle, oil, and gas will be considered in
the following paragraphs, while illumination by electricity will be
reserved for a later Chapter.

142. The Candle. Candles were originally made by dipping a wick into
melting tallow, withdrawing it, allowing the adhered tallow to harden,
and repeating the dipping until a satisfactory thickness was obtained.
The more modern method consists in pouring a fatty preparation into a
mold, at the center of which a wick has been placed.

The wick, when lighted, burns for a brief