Page:Popular Science Monthly Volume 59.djvu/253

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In considering the climatic conditions which gave to the coast of southern New England the aspect of Greenland at the present time, thought naturally turned to the antithetic phase when Greenland possessed the climate of Florida. And seemingly linked with these were other climatic variations, such as the great humidity of the Coal Measure period and the great aridity of epochs when salt and gypsum deposits accumulated; while the cause of that redness, which in several continents is characteristic of strata of certain geologic ages, might be traced to world-wide atmospheric conditions. The problem was thus greatly broadened in the scope of related phenomena, and the demands to be met by an adequate hypothesis became correspondingly complex.

The investigation upon which the hypothesis under review rests considers the physics and chemistry of the atmosphere in relation to temperature, the physics and chemistry of the ocean, the interaction of the ocean and the air, and those events of geologic history which as cause or effect may be related to the constitution of the atmosphere. It is not here proposed to review critically the several articles in which Professor Chamberlin and his associates have presented the results of profound researches. Suffice it to endeavor clearly to present an outline of their reasoning and conclusions.

The constitution of the atmosphere has long been known, and in a general way is stated for dry air as 21 parts of oxygen and 79 parts of nitrogen by volume. Argon, a newly discovered component, was formerly measured as nitrogen, and frequently there are impurities, though in small amount. There are 3 to 4 parts of carbonic acid in 10,000, and under natural conditions moisture is present in greater or less proportion. It is with these last, the carbonic acid and moisture, that the student of climatic changes has to deal chiefly.

The functions of carbonic acid and moisture in the atmosphere are threefold. They both absorb radiant heat in an unusual degree. By thus raising the temperature of the air, they both increase its capacity for moisture. And they both are chemically active.

Radiant light and heat penetrate the atmosphere to reach the solid earth, and are in part radiated back through the air into space. As the air is transparent toward light, so is it also toward heat, allowing both forms of energy to pass with moderate absorption. A photographer who compares the exposure of his plate at a considerable altitude with that near sea level roughly measures the relative strength of light at the two places and finds it less beneath the greater depth of atmosphere. The direct heat of the sun's rays is correspondingly less by the sea. The energy which the heated earth radiates back toward space is in part also absorbed by the air, which is thus warmed by the passage of rays to and from the earth.

In this absorption the mass of nitrogen and oxygen has but an