470 Dr. G. J. Stoney on Texture in Media, and on the 



vibrations according to its colour, so that the mean frequency 

 of vibration of light is about 2000 vibrations during each t*. 

 [This mean is the actual frequency of that green ray whose 

 wave-length in vacuo is 5000 tenth-metrets.] 



We may now get some insight into the physical events that 

 occur in the world into which we have passed. The pressure 

 of air in this room against the walls is, according to the 

 kinetic theory of gases, due to the walls being bombarded by 

 molecules of air as they fly about like missiles. It is an 

 elementary proposition f in the kinetic theory of gases that the 

 momentum communicated to the wall is substantially the same 

 when, as actually happens, the molecules frequently encounter 

 one another throughout the room, as it would be if the aerial 

 missiles could be divided into three equal squadrons, one of 

 which should travel uninterruptedly up and down between the 

 floor and ceiling, another squadron travelling horizontally 

 from side to side, and the third squadron from end to end of 

 the room ; and all moving with a velocity whose square is the 

 mean of the squares of all the actual velocities of molecules in 

 the room. This " velocity of mean square/' as it has been 

 called, depends on the molecular mass and on the temperature 

 of the gas ; and in the case of the air in this room it is about 

 500 metres per second J. Let us then suppose the wall to be 

 struck by one third of the molecules in this room, rushing 

 backwards and forwards between it and the opposite wall at 

 this pace ; and let us endeavour to form an estimate of how 

 often one of the superficial molecules of the wall will be sub- 

 jected to an encounter. 



The number of molecules in a cubic millimetre of the air is 

 known to be about a uno-eighteen (10 18 )§; and there are of 

 course 500,000 times this number in a column 500 metres 

 long and a square millimetre in section, i. e. there are 5 uno- 

 twenty-threes in this column. One third of all the molecules 

 in the column are the squadron that we are to regard as tra- 

 velling lengthwise, half of these advancing towards one end 

 and the other half retiring from it. It thus appears that the 

 number of molecules within the column to be taken as travel- 



5 

 ling at any instant towards one end is 77 . 20 23 . This accord- 



* See British Association Tables of Oscillation-frequency, B. A. Report 

 for 1878, p. 40. 



t See Maxwell's < Heat.' 



% Phil. Mag. 1857, xiv. p. 124; or Maxwell's 'Heat.' 



§ See footnote *, p. 469. The uno-eighteen means the number repre- 

 sented by 1 with eighteen cyphers after it. It accordingly is the same as 

 10 18 . ' 



