84 FRAGMENTS OF SCIENCE 



bodies are composed. It is the motion of these atoms, and 

 not that of any sensible parts of bodies, that the ether con- 

 veys. 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 differ- 

 ent 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 ma- 

 terially disturbing its repose. Recent experiments have 

 proved that elementary bodies, except under certain anom- 

 alous conditions, belong to the class of bad radiators. An 

 atom, vibrating in the ether, resembles a naked tuning- 

 fork vibrating in the air. The amount of motion commu- 

 nicated to the air by the 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, 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 color of 

 light. Taking a slice of white light from the sun, or from 



