On the Change of Climate during Geological Epochs. 121 



breath. The bolt-head was next connected with the experi- 

 mental tube, and the latter thoroughly exhausted, the source of 

 heat and the tube remaining the same as in previous experi- 

 ments. Communication between the bolt-head and tube was 

 then made, and closed when the barometer-gauge had sunk 15 

 inches. The needle showed a deflection of 28 0, 1, or an absorp- 

 tion of 43*3 per cent. When the whole contents of the bolt- 

 head were allowed to diffuse themselves between the vessel and 

 the tube, it was found that the mercury of the gauge had sunk 

 nearly 17 inches. The absorption found in the last observation 

 was therefore due to rather less than half the quantity of gas 

 admitted into the bolt-head. 



After the tube had been cleansed and exhausted, 0*7 inch of 

 carbonic acid was allowed to enter : this quantity of gas gave an 

 absorption of 34*7 per cent. ; but when 14-3 inches of dry air 

 were added to the gas in the tube, the absorption rose to 43*6 

 per cent. The absorption of the mixture from the bolt-head, 

 containing rather less than 0*7 inch of carbonic acid, was found 

 to be 43*3 per cent. Thus no difference exists between the ab- 

 sorption when carbonic acid and dry air are admitted into the 

 tube together, or when they are allowed to enter successively. 



In the first section of this paper some experiments are recorded 

 which determine the absorption by the carbonic acid in the atmo- 

 sphere. The amount of carbonic acid producing this absorption is 

 so extremely small, that it is impossible to measure anything like 

 its quantity with the barometer attached to the air-pump. A 

 tension of only '012 of an inch of the pure gas should theoreti- 

 cally be equal to 30 inches of common air. As yet, therefore, 

 this estimation cannot be made; though possibly it might 

 be accomplished by making use of the rectangular barometer 

 invented by Cassini and Bernoulli, and also by admitting the 

 carbonic acid after it has been mixed with air in a large receiver. 



Royal Institution, 

 July 1864. 



XIII. On the Physical Cause of the Change of Climate during Geo- 

 logical Epochs. By James Croll*. 



NO fact in geological science is better established than that 

 in former periods of our earth's history great changes 

 of climate, in so far at least as the northern portions are con- 

 cerned, must have taken place. But although there is universal 

 agreement among geologists in regard to the fact of those changes 

 having taken place, yet there is the greatest diversity of opinion 



Communicated by the Author. 



