46 THE CHEMISTRY OF THE PRIMEVAL EARTH. [IV. 



APPENDIX. 



ON THE CLIMATE OF THE EARTH IN FORMER GEOLOGICAL 

 PERIODS. 



The following note appeared in the London, Edinburgh, and Dublin Philosophical 

 Magazine for October, 1863. I subsequently found that this consequence of his dis- 

 coveries had not escaped Tyndall, who, in his Bakerian lecture for 1861 (Ibid., October, 

 1861), after showing that from its influence on terrestrial radiation all variation in the 

 amount of aqueous vapor must produce changes in climate, added, " Similar remarks 

 would apply to the carbonic acid diffused through the air, while an almost inappre- 

 ciable admixture of any of the hydro-carbon vapors would produce great effects on 

 the terrestrial rays, and corresponding changes in climate. It is not therefore neces- 

 sary to assume alterations in the density and height of the atmosphere, to account for 

 different amounts of heat being preserved to the earth at different times ; a slight 

 change in its variable constituents would account for this. Such changes, in fact, 

 may have produced all the mutations of climate which the researches of geologists 

 reveal." A letter from the author to Dr. Tyndall, in which this passage was cited, 

 appeared in the above-named magazine for March, 1864. 



THE late researches of Dr. John Tyndall on the relation of gases 

 and vapors to radiant heat are important in their bearing upon the 

 temperature of the earth's surface in former geological periods. He 

 has shown that heat, from whatever source, passes through hydro- 

 gen, oxygen, and nitrogen gases, or through dry air, with nearly the 

 same facility as through a vacuum. These gases are thus to radiant 

 heat what rock-salt is among solids. Glass, and some other solid 

 substances which are readily permeable to light and to solar heat, 

 offer, as is well known, great obstacles to the passage of radiant heat 

 from non-luminous bodies ; and Tyndall has recently shown that 

 many colorless vapors and gases have a similar effect, intercepting 

 the heat from such sources, by which they become warmed ami in 

 their turn radiate heat. Thus, while for a vacuum the absorption 

 of heat from a body at 212 F. is represented by 0, and that for dry 

 air is 1, the absorption by an atmosphere of carbonic-acid gas equals 

 90, by marsh gas 403, by olefiant gas 970, and by ammonia ].!!>:,. 

 The diffusion of olefiant gas of one-inch tension in a vacuum pro- 

 duces an absorption of 90, and the same amount of carbonic-acid 

 gas an absorption of 5.6. The small quantities of ozone present in 

 electrolytic oxygen were found to raise its absorptive pow.-r from 1 

 to 85, and even to 136 ; and the watery vapor present in the air at 

 ordinary temperatures in like manner produces an absorption of 

 heat represented by 70 or 80. Air saturated with moisture at the 



