in. the Air upon the Temperature of the Ground, 269 



reyerse would be the case (at least to a latitude of 50° from 

 the equator), if the carbonic acid diminished in amount. But 

 in both these cases I incline to think that the secondary 

 action (see p. 257) due to the regress or the progress of the 

 snow-covering would play the most important role. The 

 theory demands also that, roughly speaking, the whole earth 

 should have undergone about the same variations of tempera- 

 ture, so that according to it genial or glacial epochs must have 

 occurred simultaneously on the whole earth. Because of the 

 greater nebulosity of the Southern hemisphere, the variations 

 must there have been a little less (about 15 per cent.) than 

 in the Northern hemisphere. The ocean currents, too, must 

 there, as at the present time, have effaced the differences in 

 temperature at different latitudes to a greater extent than in 

 the Northern hemisphere. This effect also results from the 

 greater nebulosity in the arctic zones than in the neighbour- 

 hood of the equator. 



There is now an important question which should be 

 answered, namely : — Is it probable that such great variations 

 in the quantity of carbonic acid as our theory requires have 

 occurred in relatively short geological times ? The answer to 

 this question is given by Prof. Hogbom. As his memoir .on 

 this question may not be accessible to most readers of these 

 pages, I have summed up and translated his utterances which 

 are of most importance to our subject"*: — 



" Although it is not possible to obtain exact quantitative 

 expressions for the reactions in nature by which carbonic 

 acid is developed or consumed, nevertheless there are some 

 factors, of which one may get an approximately true estimate, 

 and from which certain conclusions that throw light on the 

 question may be drawn. In the first place, it seems to be 

 of importance to compare the quantity of carbonic acid now 

 present in the air with the quantities that are being trans- 

 formed. If the former is insignificant in comparison with 

 the latter, then the probability for variations is wholly other 

 than in the opposite case. 



" On the supposition that the mean quantity of carbonic 

 acid in the air reaches 0*03 vol. per cent., this number repre- 

 sents 0*045 per cent, by weight, or 0*342 millim. partial 

 pressure, or 466 gramme of carbonic acid for every cm. 2 

 of the earth's surface. Reduced to carbon this quantity 

 would give a layer of about 1 millim. thickness over the 

 earth's surface. The quantity of carbon that is fixed in the 

 living organic world can certainly not be estimated with the 



* Hogbom, Svensk kemisk Tidskrift, Bd. vi. p. 169 (1894). 

 PhiL Mag. S. 5. Vol. 41. No. 251. April 1896. U 



