L2 THE FORMS OF WATER IN 



30. As observed above, we have never seen the light 

 waves, but we judge of their presence, their position, 

 and their magnitude, by their effects. Thei lengths 

 have been thus determined, and found to vary from 



about 3-00-oo^h ^° 6 o o o o ^ n °^ an inch. 



81. But besides those which produce light, the 

 sun sends forth incessantly a multitude of waves which 

 produce no light. The largest waves which the sun 

 sends forth are 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 sift or filter the beam so as to inter- 

 cept 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 the 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-wa.ves free transmission. 

 This last separation is, however, not so perfect as the 

 first. 



33. We shall learn presently how to detach the one 

 class of waves from the other class, and to prove that 

 waves competent to light a fire, fuse metal, or burn 

 the hand like a hot solid, may exist in a perfectly dark 

 place. 



34. Supposing, then, that we withdraw, in the firsl 

 instance the large heat-waves, and allow the light- 



