76 
5. Croll’s hypothesis, named from one of its ablest advo- 
cates, has been received with much favor. It postulates a change 
in the ellipticity of the earth’s orbit along with a precession of 
the equinoxes. It is an attractive explanation and has probably 
been received with greater favor than any of the others, yet there 
have been some grave objections brought against it. It places 
the last glacial period too far away, and there does not appear 
to have been the regular recurrence that the hypothesis demands. 
Becker’s mathematical investigations of the hypothesis lead him 
to the conclusion that maximum ellipticity is the most unfavor- 
able for glacial accumulation instead of the most favorable as in- 
volved in the hypothesis. While Becker’s conclusions are based on 
mathematics advocates of the hypothesis may question some of 
his premises on which the reasoning is based. 
6. An attempt is now being made by some of the students of 
glacialogy to establish a working hypothesis for the cause of the 
glacial climate on an atmospheric basis. It is not entirely inde- 
pendent of some of the explanations already offered, nor can it 
be said to be wholly new. It attempts to put in concrete shape 
principles that have been known for many years. More than fifty 
years ago Tyndall urged that a variation in the content of CO2 
in the atmosphere was sufficient for a glacial climate, but the 
statement received scarcely more than a passing notice until quite 
recently, when it has been elaborated and presented in such detail 
and in such concrete form as to command the attention of all per- 
sons interested in the subject. 
It has long been known that CO2 and water vapor are the 
chief agents in retaining the heat radiated from the earth’s sur- 
face, but apparently no effort was made to consider the quantita- 
tive effect upon the temperature of the atmosphere by any given 
change in the content of CO2 until a few years ago when it was 
undertaken by Prof. Svante Arrhenius*. He explains that the 
air retains heat in two ways: I. The heat suffers selective dif- 
fusion as it passes through the air. 2. The gases themselves 
have the power of absorbing selectively the light and heat of cer- 
tain wave lengths. The CO2 and the water vapor have this power 
* Phil. Magazine and Journal of Science. V. 41. (Sth series.) 1896. 
