AND ETHER WAVES. 87 



combination. No change is perceptible to the eye, but 

 the keen vision of experiment soon detects the fact 

 that the perfectly transparent and highly attenuated 

 ammonia resembles pitch or lampblack in its behaviour 

 to the rays of obscure heat. 



There is probably boldness, if not rashness, in the 

 attempt to make these ultra-sensible actions generally 

 intelligible, and I may have already transgressed the 

 limits beyond which the writer of a familiar article 

 cannot profitably go. There may, however, be a rem- 

 nant of readers willing to accompany me, and for their 

 sakes I proceed. A. hundred compounds might be 

 named which, like the ammonia, are transparent to 

 light but more or less opaque — often, indeed, intensely 

 opaque — to the rays of heat from obscure sources. Now 

 the difference between these latter rays and the light- 

 rays is purely a difference of period of vibration. The 

 vibrations in the case of light are more rapid, and tl e 

 ether waves which they produce are shorter, than in 

 the case of obscure heat. Why then should the ultra- 

 red waves be intercepted by bodies like ammonia, while 

 the more rapidly recurrent waves of the whole visible 

 spectrum are allowed free transmission ? The answer I 

 hold to be that, by the act of chemical combination, 

 the vibrations of the constituent atoms of the mole- 

 cules are rendered so sluggish as to synchronise with 

 the motions of the longer waves. They resemble loaded 

 piano-strings, or slowly descending water-jets, requiring 

 notes of low pitch to set them in motion. 



The influence of synchronism between the 6 radiant ' 

 and the ' absorbent ' is well shown by the behaviour of 

 carbonic acid gas. To the complex emission from our 

 heated stove, carbonic acid would be one of the most 

 transparent of gases. For such waves defiant gas, for ex- 

 ample, would vastly transcend it in absorbing power. 



