Pity steal Theory of Secular Changes of Climate. 367 



millimetres below that after the same interval of time in the 

 first case. 



That the chemical and physical actions that take place at 

 the surface of the absorbing agent in these experiments do 

 influence the rate of absorption cannot be denied; and to what 

 an extent these actions distort the measurements as regards 

 their bearing on the interdiffusion of gases, and whether they 

 will account for the anomaly in the case of CI and S0 2 com- 

 pared with C0 2 and SH 2 , are important points for further 

 study. The employment of different alkalies for absorbing 

 these gases and the various acids for absorbing NH 3 would 

 seem to afford ample material for working out the subject ; 

 and this may form the substance of a future communication. 

 Considering, however, the result obtained by a variation of 

 the strength of the potash, it may be conjectured that neither 

 the nature of the alkali employed will influence much the rate 

 of absorption of these gases, nor the acid that of NH 3 ; but 

 these points remain to be proved by experiment. 



XLI. Examination of Mr. Alfred E. Wallace's Modification 

 of the Physical Theory of Secular Changes of Climate. By 

 James Croll, LL.D., F.E.S. 



[Continued from p. 111.] 



Part II. Geological and Palceontological Facts in relation to 

 Mr. Wallaces Modificaton of the Theory. 



MR. WALLACE'S chief, and indeed only real, modifica- 

 tion of my theory, is to the effect, as I have pointed 

 out, that the alternate phases of precession causing the winter 

 of each hemisphere to be in aphelion and perihelion each 

 10,500 years would produce a complete change of climate 

 only when a country was partially snow-clad. According to 

 his view, when the greater part of North-western Europe was 

 almost wholly buried under snow and ice, those glacial con- 

 ditions must have continued, and perhaps have even become 

 intensified, when the winter solstice moved round to perihelion, 

 instead of being replaced, as I have maintained, by an almost 

 perpetual spring. In short, Mr. Wallace's conclusion is that, 

 during the Glacial Epoch proper, a warm and equable Inter- 

 glacial Period could not have occurred. 



In the preceding part of this paper I have endeavoured to 

 show that physical principles do not warrant such a con- 

 clusion. I shall now proceed to consider what the direct 

 testimony of Geology and Palaeontology is on the subject; 

 and I believe we shall find that the facts of Geology and 



