Stoney — On Texture in Media, etc. 395 



i. e. it is one-third of the eleventhet (e) of a second of time. In this 

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

 according to its colour, so that the mean frequency of vibration of 

 light is about 2000 vibrations during each r.(/) [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 

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

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

 the air as they fly about like missiles. It is an elementary propo- 

 sition (g) in the kinetic theory of gases that the momentum com- 

 municated 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 tra- 

 velling 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 tempera- 

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

 500 metres per second. (A) 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 subjected to an encounter. 



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



molecules of certain gases in their excursions between their encounters. See Philoso- 

 phical Magazine for January, 1860, p. 32, and for July, 1860, p. 31. See also 

 Philosophical Transactions for 1866, p. 258. 



(e) 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 — - • 



(/) See British Association Tables of Oscillation-frequency, B. A. Report for 1878, 

 p. 40. 



iff) See Maxwell's "Heat." 



(h) Phil. Mag., 1857, vol. xiv., p. 124; or Maxwell's "Heat." 



