he temperature to which any matter
falls which receives no heat from any other matter. It is also sometimes
called the temperature of space, or of the ether in a state of rest, if
that is possible. All the gases have now been proved to become, first
liquid and then (most of them) solid, at temperatures considerably above
this zero.

The only way to compare the proportional temperatures of bodies, whether
on the earth or in space, is therefore by means of a scale beginning at
this natural zero, instead of those scales founded on the artificial
zero of the freezing point of water, or, as in Fahrenheit's, 32 deg. below
it. Only by using the natural zero and measuring continuously from it
can we estimate temperatures in relative proportion to the amount of
heat received. This is termed the absolute zero, and so that we start
reckoning from that point it does not matter whether the scale adopted
is the Centigrade or that of Fahrenheit.

_The Complex Problem of Planetary Temperatures._

Now if, as is the case with Mars, a planet receives only half the amount
of solar heat that we receive, owing to its greater distance from the
sun, and if the mean temperature of our earth is 60 deg. F., this is equal
to 551 deg. F. on the absolute scale. It would therefore appear very simple
to halve this amount and obtain 275.5 deg. F. as the mean temperature of
that planet. But this result is erroneous, because the actual amount of
sun heat intercepted by a planet is only one condition out of many that
determine its resulting temperature. Radiation, that is loss of heat, is
going on concurrently with gain, and the rate of loss varies with the
temperature according to a law recently discovered, the loss being much
greater at high temperatures in proportion to the 4th power of the
absolute temperature. Then, again, the whole heat intercepted by a
planet does not reach its surface unless it has no atmosphere. When it
has one, much is reflected or absorbed according to complex laws
dependent on the density and composition of the atmosphere. Then, again,
the heat that reaches the actual surface is partly reflected and partly
absorbed, according to the nature of that surface--land or water, desert
or forest or snow-clad--that part which is absorbed being the chief
agent in raising the temperature of the surface and of the air in
contact with it. Very important too is the loss of heat by radiation
from these various heated surfaces at different