78 FKA.GMENTS OF SCIENCE. 



bodies, that the ether conveys. This motion is the 

 objective cause of what, in our sensations, are light and 

 heat. An atom,' then, sending its pulses through the 

 ether, resembles a tuning-fork sending its pulses 

 through the air. Let us look for a moment at this 

 thrilling medium, and briefly consider its relation to 

 the bodies whose vibrations it conveys. Different bodies, 

 when heated to the same temperature, possess very dif- 

 ferent powers of agitating the ether : some are good 

 radiators, others are bad radiators ; which means that 

 some are so constituted as to communicate their atomic 

 motion freely to the ether, producing therein powerful 

 undulations ; while the atoms of others are unable thus 

 to communicate their motions, but glide through the 

 medium 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, re- 

 sembles a naked tuning-fork vibrating in the air. The 

 amount of motion communicated to the air by the thin 

 prongs is too small to evoke at any distance the sensa- 

 tion of sound. But if we permit the atoms to com- 

 bine chemically and form molecules, the result, in 

 many cases, is an enormous change in the power of 

 radiation. The amount of ethereal disturbance, pro- 

 duced by the combined atoms of a body, may be many 

 thousand times that produced by the same atoms when 

 uncombined. 



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 note answers 

 to the colour of light. Taking a slice of white light 

 from the sun, or from an electric lamp, and causing the 

 light to pass through an arrangement of prisms, it is 

 decomposed. We have the effect obtained by Newton, 



