Non-existence of Density in the Elemental ^Ether. 469 



meaning by the texture of a medium whatever is going on in 

 it at close quarters. 



It is, the same with the other recognized physical properties 

 of media, such as what are called gaseous laws — the laws con- 

 necting the density, temperature, and pressure of gases, the 

 laws of their diffusion, the law of viscosity, and so on. Simi- 

 larly with the properties of solid bodies : all are the outcome 

 of vast numbers of very diverse individual events that occur 

 between or within the molecules of the bodies, or between 

 them and the luminiferous aether. 



In order to penetrate to this world of individual actions, we 

 must not only descend to magnitudes that are comparable to 

 the intervals at which the centres of molecules are spaced 

 from one another, but we must also consider periods of time 

 that are not too vast in reference to the motions that go on 

 among them. A second of time is "out of all whooping" too 

 long * ; but a period which is definite and of suitable brevity 

 is that for which I have elsewhere proposed the symbol t, 

 viz. the time that light takes to advance one millimetre in vacuo. 

 The velocity of light being 30 quadrants per second, and the 

 quadrant (the length of a meridian from the Earth's equator 

 to the pole) being 10 10 millimetres, we find that 



1 one second 

 T ~3 W 1 ' 



i. e. it is one third of the eleventhet | of a second of time. In 

 this fragment of time, visible light makes from 1300 to 2600 



* See tlie Philosophical Magazine for August 1868, p. 140, footnote. 

 Readers of the paper here referred to are requested to change the square 

 of 16 in the second paragraph into the square root of 16. In that paper 

 (p. 141) I estimated the number of molecules in a cubic millimetre of a 

 gas, at atmospheric temperatures and pressures, as about a uno-eighteen 

 (10 ly ), without being aware that a similar estimate had been obtained for 

 solids and liquids by Professor Loschmidt in 1865 (Proceedings of the 

 Mathematical Section of the Academy of Vienna for October 1865, p. 405). 

 In March 1870 Sir William Thomson, doubtless also without knowing of 

 what had been done before, published a paper in 'Nature ' on the " size 

 of atoms," and arrives at substantially the same estimates as Professor 

 Loschmidt and myself. 



The earliest determination of a molecular magnitude, so far as I am 

 aware, was that made by Professor Clerk Maxwell of the mean length of 

 the " free paths " of the molecules of certain gases in their excursions 

 between their encounters. See Philosophical Magazine for January 1860, 

 p. 32, and for July 1860, p. 31 . See also Philosophical Transactions for 

 1866, p. 258. 



f The eleventhet means a unit in the eleventh place of decimals. It 



accordingly is a name for the fraction 0*000, 000, 000, 01, or TqH. 

 Phil. Mag. S. 5. Vol. 29. No. 181. June 1890. 2 O 



