IN CLOUDS ASTD RIVERS, ICE AND GLACIERS. 



87 



Fia. 1. CLOUD-BANNER OF THK AIGUILLE DU BRU (par. 64 and 227X- 



conception, which regards light a~s the pro- 

 duct of wave-motion ; but we must go far- 

 ther than ihis, and follow the conception into 

 some of its details. We have all seen the 

 waves of water, and we know they are of 

 different sizes different in length and differ- 

 ent in height. When, therefore, you are told 

 that the atoms of the sun, and of almost all 

 other luminous bodies, vibrate at diffcrci:: 

 rates, and produce waves of different sizes 

 your experience of water-waves will enable 

 you to form a tolerably clear notion of what 

 is meant. 



30. As observed above, we have never seen 

 the light- waves, but we judge of their pres- 

 ence, their position, and their magnitude, by 

 their effects. Their lengths have been thus 

 determined, and found to vary from about 

 to ^ooouth of an inch. 



31. But besides those which produce light 

 the sun sends forth incessantly a multitude 

 of waves which produce no" light. Ths 

 largest waves which the sun sen'is forth r.rc 

 of this non-luminous character, though they 

 possess the highest heating power. 



32. A common sunbeam contains waves of 

 all kinds, but it is possible to ftift or filter the 

 beam so as to intercept all its light, and to 

 allow its obscure heat to pass unimpeded. 

 For substances have been discovered which, 

 while intensely opaque to the light- waves, 

 are almost perfectly transparent to^he others. 

 On the other hand, it is possible, by the 

 choice of proper substances, to intercept in a 

 great degree the pure heat-waves, and to 

 allow the pure light-waves free transmission. 

 This last separation is, however ; not so per- 

 fect as the h'rst. 



