CLOUDS AND RIVERS, ICE AND GLACIERS. 13 



waves alone to pass. These may be concentrated by 

 suitable lenses and sent into water without sensibly 

 warming- it. Let the ligfht-waves now be withdrawn, 

 and the larger heat-waves concentrated in the same 

 manner ; they may be caused to boil the water almost 

 instantaneously. 



85. This is the point to which I wished to lead you, 

 and which without due preparation could not be under- 

 stood. You now perceive the important part played by 

 these large darkness-waves, if I may use the term, in 

 the work of evaporation. When they plunge into seas, 

 lakes, and rivers, they are intercepted close to the sur- 

 face, and they heat the water at the surface, thus 

 causing it to evaporate ; the light-waves at the same 

 time entering to great depths without sensibly heating 

 the water through which they pass. Not only, there- 

 fore, is it the sun's fire which produces evaporation, 

 but a particular constituent of that fire, the existence 

 of which you probably were not aware of. 



36. Further, it is these selfsame lightless waves 

 which, falling upon the glaciers of the Alps, melt the 

 ice and produce all the rivers flowing from the glaciers; 

 for I shall prove to you presently that the light-waves, 

 even when concentrated to the uttermost, are unable 

 to melt the most delicate hoar-frost ; much less would 

 they be able to produce the copious liquefaction observed 

 upon the glaciers. 



37. These large lightless waves of the sun, as well aa 



