The Evolution of Climates. — AIa/iso?i. 175 
Tyndall and Buff have shown, as previously noted, that solar 
light and heat-rays are largely converted, by contact with the 
planetary surface, into dark heat-rays, and are trapped; and 
that this power to trap dark heat-rays is held individually and 
collectively by the various constituents of the atmosphere. 
Now, when this heat trapping or selective absorption pro- 
cess is inaugurated upon a planet, it is no longer a heat-losing 
body, but within certain limits, it is converted into a heat- 
gathering body in space; for its rate of receipt of heat is greater 
than its rate of loss. The process has a moderate limit fixed By 
the evaporation of water, which evaporation, when excessive, 
shuts out solar energy by extensive cloud formation. 
The mean surface temperatures of the earth have thus grad- 
ually risen from the lower temperatures prevailing during past 
glacial extension. As the progress of this rise is yet being re- 
corded by the retreat of glaciers in both hemispheres and at 
all latitudes, and as it was inaugurated at a comparatively re- 
mote period in the past, the element Time has entered into the 
result as a third important factor. 
Thus the solar climates of a planet are determined by 
three prime causes or factors; first, the actual amount of heat 
and light received; second, the heat trapping power of its at- 
mosphere determining the difference between its rate of receipt 
and rate of loss of heat; third, the time these two factors or 
causes have been operating. 
In the application of this reasoning to Mars we can fix, 
in general terms, the existence and positive character of each 
cause; the logical result of the combination being that Mars 
may enjoy a milder climate than the earth. 
As regards the first, the amount of heat and light Mars 
receives is readily calculated to be about 0.43 that received 
by the earth. 
The existence and operation of conditions tending to ab- 
stract and retain solar energy is made manifest by the deficien- 
cy of blue rays and the excess of red and orange ravs in the 
solar light Mars transmits to us, the color of this transmitted 
light indicating that the Martian atmosphere has the power of 
abstracting and retaining those rays which are most readily 
trapped, or selectively absorbed by the atmosphere of the 
earth. This deficiency in the rays most readily trapped indi- 
