insignificant part. They are nearly perfectly transparent to heat. Carbonic acid and moisture are the effective constituents, which thicken, as it were, the atmospheric blanket, and being warmed in turn keep warm the earth. If they are decreased the blanket becomes thin and the surface grows cool.
Tyndall first suggested that a lessening of the proportion of carbonic acid might suffice to bring on the cold climate of a glacial epoch. He was followed by several investigators who determined more accurately the parts played by carbonic acid and by moisture, Austrian, German and American scientists competing in the study. In 1896, Dr. Arrhenius, a Swedish physicist, reached definite quantitative estimates of the effects. Employing values for the radiant heat of the full moon at different heights above the horizon, measured by Langley, he computed the heat absorbed by the atmosphere. By elaborate calculations he determined that a decrease of carbonic acid in the atmosphere to an amount ranging from 55 to 67 per cent, of the present content would reduce the average temperature 4 or 5 degrees C, which would bring on a glacial epoch, whereas an increase of carbonic acid to an amount two or three times the present content would elevate the average temperature 8 or 9 degrees C, and bring on a mild climate in high latitudes.
The effects of relatively absorbent or transparent atmospheres are not direct and uniform. They vary with the angle of incidence of the sun's rays and, therefore, with latitude, with seasons, and with day and night. They differ with altitude above the earth's surface; they are unlike on land and sea. But in general result the effect of greater absorptive power is to equalize all differences due to geographical and astronomical relations, whereas that of a relatively transparent condition is to accentuate them.
The physicist having thus indicated a possible solution, the further task of framing a working hypothesis was the geologist's. Chamberlin says: "There are hypotheses and working hypotheses. . . . General suggestions of a possible cause do not reach the dignity of working hypotheses until they are given concrete form, are fitted in detail to the specific phenomena and are made the agents of calling into play effective lines of research." In his attempt to frame a working hypothesis of the cause of glacial periods on an atmospheric basis, he has nobly met the requirements of his own definition under the difficult conditions imposed by the phenomena of glaciation. However the resulting working hypothesis may hereafter be modified by further research, its presentation must always stand as an example of the highest scientific effort.
Let us briefly review the requirements of the task. The fundamental postulate of the hypothesis is that variations of the atmospheric
