276 KELVIN 



is as easy to understand that number as any number like 

 2, 3, or 4. Consider now what that number means and what 

 we are to infer from it. What force is there in the space be- 

 tween my eye and that light ? What forces are there in the 

 space between our eyes and the sun, and our eyes and the re- 

 motest visible star ? There is matter and there is motion, but 

 what magnitude of force may there be? 



I move through this " luminiferous ether " as if it were 

 nothing. But were there vibrations with such frequency 

 in a medium of steel or brass, they would be measured by 

 millions and millions and millions of tons' action on a square 

 inch of matter. There are no such forces in our air. Comets 

 make a disturbance in the air, and perhaps the luminiferous 

 ether is split up by the motion of a comet through it. So 

 when we explain the nature of electricity, we explain it by 

 a motion of the luminiferous ether. We cannot say that 

 it is electricity. What can this luminiferous ether be? It 

 is something that the planets move through with the greatest 

 ease. It permeates our air; it is nearly in the same con- 

 dition, so far as our means of judging are concerned, in 

 our air and in the inter-planetary space. The air dis- 

 turbs it but little; you may reduce air by air-pumps to 

 the hundred thousandth of its density, and you make little 

 effect in the transmission of light through it. The .lumi- 

 niferous ether is an elastic solid, for which the nearest 

 analogy I can give you is this jelly which you see, 6 arid the 

 nearest analogy to the waves of light is the motion, which 

 you can imagine, of this elastic jelly, with a ball of 

 wood floating in the middle of it. Look there, when with 

 my hand I vibrate the little red ball up and down, or when I 

 turn it quickly round the vertical diameter, alternately in 

 opposite directions ; that is the nearest representation I can 

 give you of the vibrations of luminiferous ether. 



Another illustration is Scottish shoemakers' wax or Bur- 

 gundy pitch, but I know Scottish shoemakers' wax better. 

 It is heavier than water, and absolutely answers my purpose. 

 I take a large slab of the wax, place it in a glass jar filled 

 with water, place a number of corks on the lower side and 



6 Exhibiting a large bowl of clear jelly with a small red wooden ball embedded 

 in the surface near the centre. 



