the vibrations of this , ether being communicated to material objects,
gives them a more rapid vibration, that is, increases their temperature.
If we stand in front of a fire, we experien'ce a sensation of warmth
which is not due to the temperature of the air, for if a screen be interposed,
the sensation immediately disappears, which would not be the
case if the surrounding air had a high temperature. Hence bodies can
send out rays which excite heat, and which penetrate through the air
without heating it, as rays of light through transparent bodies. Heat
thus propagated is said to be radiated; and the term, '¡¡ay of heat or
calorific ray is used in the same sense as ray of light is used. In a
homogeneous medium, radiation takes place in a right line. The
intensity is less the greater the obliquity of the rays, with respect to
the radiating surface. Thermal rays falling upon a body are divided
into two parts, one of which penetrates the body, while the other
rebounds—that is, it is reflected from the surface. We live here at the
bottom of an aerial ocean, which is to a remarkable degree permeable
to the sun’s rays of light and heat, and is but little if at all affected by
the direct action of this heat. But the rays when they fall upon the
earth heat its surface, and when upon the ocean evaporate the water.
The air, in direct contact with the heated surface of the earth, which
surface is reflecting back and radiating the solar heat, becomes warmed,
and it is a principle of physics that hot air is lighter than cold air.
This warm air then rises and flows northwards towards the poles, ■
while the cold air from the Arctic regions flows southward to the
equator. This is the principle of trade-winds, hot winds blowing from
the equator to the poles, and cold winds from the poles to the equator.
This is one of the causes which tends to modify the rigour of the Polar
night. Another is that the heat stored up by the earth’s surface during
the summer months is radiated slowly, and thus the atmosphere is
warmed by gradual radiation from the earth’s surface of solar heat
received during the summer. Absence of heat or “ no heat,” as the
“ Header of Colburn's New Monthly also” puts it, has never been
observed; and as there is never an absence of motion in matter, insomuch
as molecular motion always exists, there is probably no total
absence of h e a t: for heat is motion. During the day, or Polar
summer, the ground receives more heat than it radiates into space, and
the temperature rises. The reverse is the case during the night; the
heat which the earth loses by radiation is no longer compensated for,
and, consequently, a fall of temperature takes place. In tropical Bengal
this nocturnal cooling is used for the manufacture of ice. Large flat
vessels containing water are placed on non-conducting substances, such
Reminiscences and Relics. 35
as straw or dry leaves. In consequence of the radiation the water
freezes, even when the temperature of the air is 60° Fahr. If the heat
did not come from the sun, but were common to the atmosphere, there
would be no evaporation, and therefore no clouds. It will now be
seen how absurd a fallacy it is to talk of “ heat dormant in the
atmosphere. But as a convincing proof that heat comes from the
sun, the following experiment is useful:—When a solar ray admitted
through an aperture in a dark room is concentrated on a prism of rock
salt, and then^ after emerging from the prism, is received on a screen,
it will be found to present a band of colours in-the following order :
red, orange, yellow, green, blue, and violet. This is the light and heat
spectrum. (It demonstrates the fact that white light is composite.)
If, now, a narrow and delicate thermo-pile or thermometer be placed
successively on the space occupied by each of the colours, it will be
scarcely affected on the violet; but in passing over the other colours it
will indicate a gradual rise of temperature, which is greatest at the red.
If the thermometer be now moved in the same direction beyond the
limits of the luminous spectrum, the temperature will gradually rise
for a considerable space beyond the light spectrum. This is the heat
spectrum. _ I t is therefore seen that the sun’s light consists of rays of
light and heat of different rates of vibration \ by their passage through
the prism they are unequally broken or refracted | those with shorter
wave-lengths are the most refrangible. This experiment will also hold
good for all sources of heat, as candles, the moon, electric light, &c.
The next point in the letter of the “Beader of Colburn’s New Monthly
also is the vacuum, which he says I introduced into my notes, “ to
strengthen” my “ ratiocination that heat comes from the sun.” I am
sure I did no such thing, as the observation has no connection with
the matter. I merely stated a fact, that heat passes through a vacuum.
Your correspondent tells me to “ experiment with a vacuum, enclosed
both top and sides with glass,” and adds (somewhat after the fashion
of the “ March Hare” in “ Alice in Wonderland,” who, when the
“ Hatter ” says that he knew butter would not suit the works of his
watch, replies, “ It was the best butter, you know,”) that, “ it must be
plate glass to resist the pressure.” How does one enclose a vacuum
“ top and sides,” or which is the top of a vacuum and where are the
sides; and does your correspondent leave out the bottom altogether 1
However this may be, he concludes from this remarkable experiment
performed with a vacuum, with a top and sides,” that “ the atmosphere,
the mother of heat, as far as possible is drawn out of it, and he
will, referring to myself, “ find that the heat is so wonderful within
D 2