Ixvii 



cide with these vibrations, whilst the other waves not concurring with 

 them, would be less retarded. 



This enables us to comprehend why some of the waves of light will pass 

 through certain substances, and maintain their respective periods of vibra- 

 tion and consequent energy beyond them, whilst others are taken up and 

 absorbed by the vibrations of these substances, and therefore can give no 

 evidence of existence beyond such intercepting media. 



When a beam of light falls upon a body which absorbs a portion only 

 of those rays which excite vision, the others which pass through will pro- 

 duce the effect of color. When all the rays of light play upon the retina, 

 the sense of white or colorless light is produced. White may be pro- 

 duced also by several pairs of colored lights. Yellow and blue are exam- 

 ples of colored lights which combine to produce white. Such colors are 

 termed complementary colors. If instead of blue and yellow lights we 

 combine pigments, the result is quite different. This would produce green. 

 The waves of light and the obscure heat rays both pass freely through 

 a crystal of rock-salt, the periods of its molecular vibration being such 

 as scarcely to intercept them at all. Hence rock-salt is said to be trans- 

 parent to the rays of light and transcalescent to the rays of heat. 



Clear glass is only transparent to the rays of light, while to the rays 

 of heat it is nearly opaque. Hence a glass screen placed in front of a 

 fire permits the light rays to pass freely, but intercepts the heat rays. 

 Rock-salt would not be heated sensibly by the passage of either class of 

 rays. The obscure rays of heat arouse molecular vibrations in the screen 

 and are absorbed, and hence persons seated beyond the screen are pro- 

 tect ed from those rays. The glass becomes heated by absorbing them, 

 but this heat is radiated from it not only in the direction of the rays but 

 in all other directions, so that those behind the glass only receive a 

 small portion of the heat which is radiated. 



Ice will transmit very freely the waves of both light and heat through 

 it ; and Faraday has even exploded gun-powder in the focus of a lens of ice. 

 The air we breathe is almost perfectly transparent to the rays of heat 

 and light, but it is quite different with the invisible vapor of water held in 

 suspense in the air. The heat rays are absorbed to a large extent by this 

 vapor, while the rays of light pass freely through it. Hence in climates 

 favored by the sea-breeze, the air is so saturated with moisture that while 

 the light of the sun is undimmed by it, his intense heat rays are to a 

 large extent intercepted. At night when the earth would rapidly radiate 

 the heat received during the day, this vapor interposes a protecting shield 

 again, and prevents the intensely cold nights that would result from this 

 loss of heat. Thus countries over which the trade winds pass have equa- 

 ble climates and experience but little difference in the temperature of 

 night and day. In the elevated regions of high mountains this vapor is 

 condensed by the cold and becomes visible to us in fogs and clouds, and 

 finally falls in the form of mist, rain and snow. The atmosphere, when 

 thus relieved, permits the heat rays to pass through it so freely that they 

 attack the human form with such intensity, even amidst snow and ice in 



