the Luminiferous Ether. 235 



of the magnitude of these particles. The best microscopes will 

 detect particles ^^ of an inch in diameter ; yet " we are 

 here dealing with infinitesimals, compared with which the test- 

 objects of the microscope are literally immense " *. By means 

 of the spectroscope we can detect 2 oo,ooo,ooo °f a g ram of sodium ; 

 so that the atom of this metal must be smaller than this. Also, 

 " the number of molecules in a cubic millimetre of atmospheric 

 air is about a unit-eighteen (10 18 ) " f — that is, one million 

 billions ! (A billion is a million times a million.) With a 

 wave-length of ^ of a millimetre, we cease to have any lumi- 

 nous effect, but we . still possess a faint photographic effect. 

 We therefore see that the shortest waves in the spectrum are 

 of immense length when compared with the size of the mole- 

 cules of a body. With regard, then, to the size of the mole- 

 cule, we can have no doubt that it is sufficiently minute to 

 be fully able to oscillate and produce waves of the size of those 

 of light. But can these molecules oscillate with sufficient 

 rapidity ? In an article on " Polarization Stress in Gases " 

 (Phil. Mag. Dec. 1878), Mr. Gr. J. Stoney supplies data which 

 will enable us to answer this question. At common tempe- 

 ratures the average velocity of the molecules of air may be 

 taken as 500 metres per second. The molecules meet with so 

 many encounters that the direction of the path of each is 

 changed 10,000,000,000 times a second. We have, then, that 

 in one movement the particle travels 2>00 q 00 of a metre, or 

 — of a millimetre ; and it makes this movement in 10 ooq> q 00 000 

 of a second. Now we have seen that the length of the 

 wave of the extreme chemical ray is ^- of a millimetre ; con- 

 sequently we find that the molecule of air travels through a 

 distance which is more than twice as long as the length of this 

 particular wave in this fraction of a second. The time of one 

 oscillation of the molecules composing the mean chemical rays 

 may be taken as 800>000)000)000 of a second. Thus, in 80 times as 

 long as the time occupied by a molecule in one oscillation the 

 molecule of air has travelled through a distance twice as long 

 as that of the whole wave-length. The distance moved through 

 by a wave would be underestimated at a million times the 

 distance moved through by a molecule composing that wave ; 

 consequently we see that our air-particles move with a far 

 higher velocity than that required by the shortest waves of the 

 spectrum. 



* Tyndall, ( Scientific Use of Imagination/ p. 25. 

 t Johnstone Stoney, Phil. Mag. December 1878. 



