Condensational-raref "actional Waves in Gases §c. 497 



have a propagation outwards of disturbance due to viscosity, 

 very interesting in itself ; but we should have no motion 

 that we know of appropriate to our present subject until we 

 rise to frequencies of 10 9 , 10 X 10 12 , 400 x 10 12 , 800 x 10 12 , or 

 3000 x 10 12 , when we should have radiant heat, or visible 

 light, or ultra-violet light proceeding from the outer surface 

 of the shell, as it were from a point-source of light at the 

 centre, with a character of polarization which we shall 

 thoroughly consider a little later. But now let our massless 

 shell be embedded far in the interior of a vast mass of glass, 

 or metal, or rock, or of any homogeneous elastic solid, firmly 

 attached to it all round, so that neither splitting away nor 

 tangential slip shall be possible. Purely distortional waves 

 will spread out in all directions except the axial. Suppose, to 

 fix our ideas, we begin with vibrations of one-second period, 

 and let the elastic solid be either glass or iron. At distances 

 of hundreds of kilometres (that is to say, distances great in 

 comparison with the wave-length and great in comparison 

 with the radius of the shell), the wave-length will be approxi^ 

 mately 3 kilometres *. Increase the frequency now to 1000 

 periods per second : at distances of hundreds of metres the 

 wave-length will be about 3 metres. Increase now the fre- 

 quency to 10 6 periods per second ; the wave-length will be 

 3 millim., and this not only at distances of several times the 

 radius of the shell, but throughout the elastic medium from 

 close to the outer surface of the shell ; because the wave-length 

 now is a small fraction of the radius of the shell. Increase 

 the frequency further to 1000 x 10 6 periods per second ; the 

 wave-length will be 3xl0~ 3 of a millim., or 3 mikroms f , 



* Math, and Phys. Papers, toL iii. art. civ. p. 522. 



t For a small unit of length Langley, fourteen years ago, used with 

 great advantage and convenience the word " mikron " to denote the 

 millionth of a metre. The letter n has no place in the metrical system, 

 and I venture to suggest a change of spelling to " mikrom " for the 

 millionth of a metre, after the analogy of the English usage for millionths 

 (mikrohm, mikro-ampere, mikrovolt). For a conveniently small corre- 

 sponding unit of time I further venture to suggest "niichron " to denote 

 the period of vibration of light whose wave-length in aether is 1 mikrom. 

 Thus, the velocity of light in aether being 3x10* metres per second, the 

 michron is |xl0 — M of a second, and the velocity of light is 1 mikrom 

 of space per michron of time. Thus the frequency of the highest ultra- 

 violet light investigated by Schumann (-1 of a mikrom wave-length, 

 Sitzungsber. d. k. Gesellsch. d. Wissensch. zu Wien, cii. pp. 415 & &2o, 

 1893) is 10 periods per michron of time. The period of sodium light 

 (mean of lines D) is '589212 of a michron ; the periods of the " Rest- 

 strahlen ; ' of rocksalt and svlvin found hy Rubens and Aschkinass ( Wied. 

 Ann lxv. (1898) p. 241) are 51*2 and 61*1 michrons respectively. 



No practical inconvenience can ever arise from any possible confusion, 

 or momentary forgetfulness, in respect to the similarity of sound between 

 michrons of time and mikroms of space. — K, 



