president's address. 29 



luminous suggestion, wliich should receive the most careful study 

 from all interested in the great questions of earth history, and to 

 which I am much indebted.* 



The idea that glaciation over a wide range of the earth's 

 surface could be caused by removal of carbon dioxide and con- 

 currently of water vapour fnm the atmosphere, is by no means 

 novel. It appears to have been first suggested by Professor 

 Tyndall, who was led thereto by the result of his observations on 

 the thermal properties of gases and vapours. Tyndall found that 

 while simple gases such as nitrogen and oxygen, which constitute 

 the bulk of the existing atmosphere, are extremely transparent 

 to the entire solar emanation of light and heat, compound gases 

 like carbon dioxide, marsh gas, ammonia, &c., and the vapours of 

 water, alcohol, essential oils, &c., differentiate between the 

 various wave lengths, and, while allowing some to pass as freely 

 as do the simple gases, offer greater resistance to the passage of 

 others. The waves which are unable to pass through the com- 

 pound gases and vapours are the long obscure ones in the ultra- 

 led, while the shorter waves above this pass freely. Even in 

 very moderate thickness such gases are able to effectually bar 

 the progress of considerable quantities of obscure heat. 



As a matter of fact no sharp distinction can be drawn between 

 heat and light, the waves of the former passing insensibly into the 

 latter as we progress along the spectrum. Light waves differ 

 from one another and from those of heat merely in their length, 

 and, as all progress through space at equal rates, it follows that 

 the shorter waves give a greater number of impacts to a receiving- 

 surface in unit time, than do the longer. The longest waves, 

 giving the fewest impacts in unit time, are the obscure heat rays 

 of the ultra-red, and as we pass on towards the visible spectrum 

 the waves become shorter and shorter, and constitute what is 

 commonly called radiant heat. 



Professor Tyndall was the first to put the matter of the selective 

 action of gases upon radiant energy on a proper foundation, and 



* Jour. Geol. Vols. v. and vii., 1897 and 1899. 



