624 REVIEWS 
Professor Arrhenius to show the exact effect of any given change in 
the carbon dioxide content of the atmosphere upon the surface tem- 
perature of the earth. 
The essential work of the physicist and mathematician having now 
been done by him, it remains for the geologist .to investigate the various 
sources of supply and depletion of carbon dioxide and to determine if 
possible if there have been any variations of such an order of magnitude 
as to produce the results observed. 
Professor Arrhenius explains in his papers that the air retains heat 
(light and dark) in two different ways: (1) The heat suffers a selective 
diffusion as it passes through the air. This is greatest for the rays 
having short wave-lengths (ultra-violet) and insensible for those of long 
wave-lengths which form the chief part of the radiation of a body of the 
temperature of the earth, viz.,15° C. (2) The gases themselves have 
the power of absorbing selectively the light and heat of certain wave- 
lengths. The carbon dioxide and the water vapor have this power of 
selective absorption to a far greater extent than the oxygen, nitrogen 
or argon, and this absorption is not distributed evenly throughout the 
spectrum but occurs in certain definite bands which are best developed 
in the ultra-red portion which represents the rays with long wave-lengths 
such as are given off by bodies with a low temperature. 
There are two ways in which to measure the amount of the heat 
absorption by the carbon dioxide and the water vapor: (1) by measur- 
ing directly the amount of heat absorbed by such quantities of these 
gases as they appear in the atmosphere, and at a temperature of 
15° C., and (2) by measuring the amounts of heat received from the 
full moon at different heights above the horizon. ‘The amount of car- 
bon dioxide through which the rays pass is evidently a function of 
altitude of the moon above the horizon, while that of the water vapor 
depends both upon the altitude and the humidity of the air. 
Professor Arrhenius takes the second method and from Professor 
Langley’s observations on the heat received from the full moon at 
various altitudes above the horizon he calculates the amount of heat 
absorbed by the two gases by an atmosphere having the present average 
amount of carbon dioxide and the average amount of water vapor, 
viz., ten grains per cubic meter at the earth’s surface. The full moon 
has, however, a surface temperature of 100°, and he introduces the 
corrections necessary to apply the above to a body with the temper- 
atumerOLenigie Ke: 
