AND ETHER WAVES. 81 



physical philosopher for the most part resides. Science 

 has been defined as ' organised common sense,' by whom 

 I have forgotten ; but, unless we stretch unduly the 

 definition of common sense, I think it is hardly applica- 

 ble to this world of molecules. I should be inclined to 

 ascribe the creation of that world to inspiration, rather 

 than to what is currently known as common sense. For 

 the natural-history sciences the definition may stand — 

 but hardly for the physical and mathematical sciences. 

 The sensation of light is produced by a succession of 

 waves which strike the retina in periodic intervals ; and 

 such waves, impinging on the molecules of bodies, agitate 

 their constituent atoms. These atoms are so small 

 and, when grouped to molecules, are so tightly clasped 

 together, that they are capable of tremors equal in 

 rapidity to those of light and radiant heat. To a mind 

 coming freshly to these subjects, the numbers with 

 which scientific men here habitually deal must appear 

 utterly fantastical; and yet, to minds trained in the, 

 logic of science, they express most sober and certain 

 truth. The constituent atoms of molecules can vibrate 

 to and fro millions of millions of times in a second. 

 The waves of light and of radiant heat follow each other 

 at similar rates through the luminiferous ether. Fur- 

 ther, the atoms of different molecules are held together 

 with varying degrees of tightness — they are tuned, as it 

 were, to notes of different pitch. Suppose, then, light- 

 waves, or heat-waves, to impinge upon an assemblage 

 of such molecules, what may be expected to occur? 

 The same as what occurs when a piano is opened and 

 sung into. The waves of sound select the strings which 

 respectively respond to them — the strings, that is to 

 say, whose rates of vibration are the same as their own. 

 Of the whole series of strings these only sound. The 

 vibratory motion of the voice, imparted first to the 



