428 Scientific Proceedings, Royal Dublin Society. 
Absorptive power of gases for non-luminous heat. 
LAN ie ty. : : é ] 8. Carbonic acid - : 90 
2. Oxygen : : : 1 9. Nitrous oxide ; , 355 
3. Nitrogen . : - 1 10. Sulphuretted hydrogen - 890 
4, Hydrogen C : é 1 11. Marsh gas - - 403 
5. Chlorine < . ¢ a) 12. Sulphurous acid « “) LO 
6. Hydrochloric acid - 5 13. Olefiant gas : s a10 
7. Carbonic oxide - 5 8D 14. Ammonia ; : - 1195 
The experiments of Tyndall also establish the fact that aqueous 
vapour has a powerful absorbent action upon heat of low refrangi- 
bility ; varying from 30 times to 70 times the effect of pure dry air. 
It is quite certain that in Paleozoic times, the atmosphere 
contained a very large proportion of carbonic acid, which has 
since disappeared ; and it is the opinion of many geologists, that 
as late ay Miocene times, there was much more aqueous vapour 
in the atmosphere than at present. Both of these conditions, one 
certain, and the other probable, would increase the effect of the 
atmosphere, regarded as a blanket, to keep in the sun-heat 
received ;—for we must observe that while carbonic acid and 
aqueous vapour would produce but little reduction in the amount 
of lwminous sun-heat received, as compared with pure, dry, air ; 
they would present a formidable obstacle to non-lwminous heat 
escaping by radiation from the earth’s surface into the cold of star- 
space. 
On the whole, the following appear to me to be the most 
probable conclusions at which we can arrive as to the causes of 
former geological climates :— 
Ist. We must reject any solution based upon a change of 
position, either in space or within the earth’s body, of the axis of 
rotation, within the limits of geological time. 
2nd. We must reject any solution based upon the secular 
cooling of the earth (with a fixed axis of rotation), regarded as 
the sole and immediate cause of the change of climate. 
3rd. The chief factor in changes of geological climate appears 
to have been the slow secular cooling of the sun, in consequence 
of which the earth’s surface cooled gradually down.* 
* A hot body placed in a cold space would cool down, as the schoolmen would say, 
immediately ; but a body deriving its heat from a cooling fire, would cool down mediately. 
The earth has done both, but its chief cause of cooling has been the diminishing heat of 
the sun. 
