RADIANT HEAT AND ITS RELATIONS. 217 



therein powerful undulations ; while others are unable 

 thus to communicate their motion, but glide through the 

 ether without materially disturbing its repose. Recent 

 experiments have proved that elementary bodies, except 

 under certain anomalous conditions, belong to the class of 

 bad radiators. An atom vibrating in the ether resembles 

 this naked tuning-fork vibrating in the air. The amount 

 of motion communicated to the air by these thin prongs is 

 too small to evoke at any distance the sensation of sound. 

 But if we permit the atoms to combine chemically and 

 form molecules, the result in many cases is an enormous 

 change in the power of radiation. The amount of ethereal 

 disturbance produced by the combined atoms of a body 

 may be many thousand times that produced by its constitu- 

 ent atoms when uncombined. The effect is roughly typi- 

 fied by this tuning-fork when connected with its resonant 

 case. The fork and its case now swing as a compound 

 system, and the vibrations which were before inaudible, are 

 now the source of a musical sound so powerful that it 

 might be plainly heard by thousands at once. The fork 

 and its case combined may be roughly regarded as a good 

 radiator of sound. 



The pitch of a musical note depends upon the rapidity 

 of its vibrations, or, in other words, on the length of its 

 waves. Now, the pitch of a no'te answers to the color of 

 light. Taking a slice of white light from the beam of an 

 electric lamp, I cause that light to pass through an arrange- 

 ment of prisms. It is decomposed, and we have the effect 

 obtained by Newton, who first unrolled the solar beam into 

 the splendors of the solar spectrum. At one end of this 

 spectrum we have red light, at the other violet, and be- 

 tween those extremes lie the other prismatic colors. As 

 we advance along the spectrum from the red to the violet, 

 the pitch of the light — if I may use the expression — height- 

 ens, the sensation of violet being produced by a more rapid 

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