274 Prof. S. Arrhenius on the Influence of Carbonic Acid 



opinion that a change in the transparency of the atmosphere 

 would possibly give the desired effect. According to his 

 calculations, " a lowering of this transparency would effect a 

 lowering of the temperature on the whole earth, slight in the 

 equatorial regions, and increasing with the latitude into the 

 70th parallel, nearer the poles again a little less. Further, 

 this lowering would, in non-tropical regions, be less on the 

 continents than on the ocean and would diminish the annual 

 variations of the temperature. This diminution of the air's 

 transparency ought chiefly to be attributed to a greater 

 quantity of aqueous vapour in the air, which would cause 

 not only a direct cooling but also copious precipitation of 

 water and snow on the continents. The origin of this 

 greater quantity of water- vapour is not easy to explain." De 

 Marchi has arrived at wholly other results than myself, because 

 he has not sufficiently considered the important quality of 

 selective absorption which is possessed by aqueous vapour. 

 And, further, he has forgotten that if aqueous vapour is sup- 

 plied to the atmosphere, it will be condensed till the former 

 condition is reached, if no other change has taken place. As 

 we have seen, the mean relative humidity between the 40th 

 and 60th parallels on the northern hemisphere is 76 per cent. 

 If, then, the mean temperature sank from its actual value + 5*3 

 by 4°-5° C, i. e. to +1*3 or +0*3, and the aqueous vapour 

 remained in the air, the relative humidity would increase to 

 101 or 105 per cent. This is of course impossible, for the 

 relative humidity cannot exceed 100 per cent, in the free air. 

 A fortiori it is impossible to assume that the absolute 

 humidity could have been greater than now in the glacial 

 epoch. 



As the hypothesis of Croll still seems to enjoy a certain 

 favour with English geologists, it may not be without interest 

 to cite the utterance of De Marchi on this theory, which 

 he, in accordance with its importance, has examined more in 

 detail than the others. He says, and I entirely agree with 

 him on this point : — " Now I think I may conclude that from 

 the point of view of climatology or meteorology, in the 

 present state of these sciences, the hypothesis of Croll seems 

 to be wholly untenable as well in its principles as in its 

 consequences " *. 



It seems that the great advantage which Croll's hypothesis 

 promised to geologists, viz. of giving them a natural chro- 

 nology, predisposed them in favour of its acceptance. 

 But this circumstance, which at first appeared advantageous, 

 seems with the advance of investigation rather to militate 



* Pe Marchi, I. c. p. 166. 



