244 POPULAR SCIENCE MONTHLY. 



insignificant part. They are nearly perfectly transparent to heat. Car- 

 bonic 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 car- 

 bonic 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 esti- 

 mates 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 car- 

 bonic 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. . . . Gen- 

 eral suggestions of a possible cause do not reach the dignity of work- 

 ing 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 diffi- 

 cult 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 funda- 

 mental postulate of the hypothesis is that variations of the atmospheric 



