in the Air upon the Temperature of the Ground. 273 



activity, whilst other epochs have been marked by a com- 

 parative quiescence of the volcanic forces. It seems there- 

 fore probable that the quantity of carbonic acid in the air has 

 undergone nearly simultaneous variations, or at least that 

 this factor has had an important influence. 



" If we pass the above-mentioned processes for consuming 

 and producing carbonic acid under review, we find that they 

 evidently do not stand in such a relation to or dependence on 

 one another that any probability exists for the permanence 

 of an equilibrium of the carbonic acid in the atmosphere. 

 An increase or decrease of the supply continued during 

 geological periods must, although it may not be important, 

 conduce to remarkable alterations of the quantity of carbonic 

 acid in the air, and there is no conceivable hindrance to 

 imagining that this might in a certain geological period have 

 been several times greater, or on the other hand considerably 

 less, than now." 



As the question of the probability of quantitative variation 

 of the carbonic acid in the atmosphere is in the most decided 

 manner answered by Prof. Hogbom, there remains only one 

 other point to which I wish to draw attention in a few words, 

 namely : Has no one hitherto proposed any acceptable ex- 

 planation for the occurrence of genial and glacial periods ? 

 Fortunately, during the progress of the foregoing calcula- 

 tions, a memoir was published by the distinguished Italian 

 meteorologist L. De Marchi which relieves me from answer- 

 ing the last question*. He examined in detail the different 

 theories hitherto proposed — astronomical, physical, or geo- 

 graphical, and of these I here give a short resume'. These 

 theories assert that the occurrence of genial or glacial epochs 

 should depend on one or other change in the following cir- 

 cumstances : — 



(1) The temperature of the earth's place in space. 



(2) The sun's radiation to the earth (solar constant). 



(3) The obliquity of the earth's axis to the ecliptic. 



(4) The position of the poles on the earth's surface. 



(5) The form of the earth's orbit, especially its eccentricity 



(Croll). 



(6) The shape and extension of continents and oceans. 



(7) The covering of the earth's surface (vegetation). 



(8) The direction of the oceanic and aerial currents. 



(9) The position of the equinoxes. 



De Marchi arrives at the conclusion that all these hypotheses 

 must be rejected (p. 207). On the other hand, he is of the 



* Luigi De Marchi : Le cause delV era glaciale, premiato dal R. Istituto 

 Lombardo, Pavia, 1895, 



