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assumed that as early as Paleozoic times the atmosphere had 
some such constitution as it possessed in later geological times, 
and that its history has been a contest between the agencies 
of atmospheric supply and the agencies of atmospheric deple- 
tion, and that the constitution of the atmosphere at any time 
has been dependent upon the relative rates of supply and deple- 
tion, then the hypothesis may be entertained quite independ- 
ently of all views of the origin of the earth and the atmosphere 
and of internal dynamics. 
Previons advocacy of an atmospheric hypothesis.—The general 
doctrine that the glacial periods may have been due to a change 
in the atmospheric content of carbon dioxide is not new. It 
was urged by Tyndall a half century ago and has been urged by 
others since. Recently it has been very effectively advocated 
by Dr. Arrhenius,’ who has taken a great step in advance of his 
predecessors in reducing his conclusions to definite quantitative 
terms deduced from observational data.* The great labor 
involved in this and the specific results springing from it place 
his contribution on a much higher plane than the general sug- 
gestions of those who had preceded him. Valuable as these 
general suggestions were, they must still be regarded as falling 
much short of working hypotheses, since no attempt was made 
to show that changes in the content of carbon dioxide of such 
a degree as would be compatible with the continuity of life and 
with other limiting geological conditions were quantitatively 
competent to produce the effects assigned them; nor were 
modes of inquiry into this essential matter suggested. It is one 
thing to point out a theoretical causa vera, and quite another 
thing to give good reasons for believing that it is quantitatively 
sufficient, and to open lines of inquiry for demonstrating that it 
is so. This Dr. Arrhenius has done and apparently with great 
success. While his results are doubtless to be regarded as sub- 
ject to modification when more full and exact data are at hand, 
tOn the Influence of Carbonic Acid in the Air upon the Temperature of the 
Ground, by SVANTE ARRHENIUS, Phil. Mag., April 1896, pp. 237-276. 
2 See review of his paper in this number of the JOURNAL, p. 623. 
