608 Stoney — Cause of Double Lines in Spectra. 



Another observation of general application seems worth making here. Each 

 molecule of gas at atmospheric pressures and temperatures, meets with about 7000 

 encounters in the millionth of a second, and of course those which fall to the lot of 

 one molecule must happen under a great variety of circumstances. Moreover, 

 immense numbers of these molecules are present, something like a thousand 

 millions* in every cubic micron of air ; while in the liquid state they are still more 

 numerous, about a thousand times as many of the gaseous molecules being now 

 crowded into each cubic micron. They are besides now jostled almost without 

 intermission, instead of each encountering its neighbom-s only at intervals, as in a 

 gas. There are therefore abundant chances for extremely rare circumstances to 

 occur in their struggles with one another, at what we should deem very short 

 intervals of time and space ; and it is probable that many important chemical and 

 physiological effects that apjiear to us to take place with even explosive prompti- 

 tude, have in reality to wait long (from the molecular standpoint) for their 

 appropriate opportunity to arise. 



* See Phil. Mag. for August, 1868, top of page 141. Readers of the Paper here referred to are 

 requested to change the square of 16, at the end of the second paragraph on p. 134, into the square root 

 of 16. The micron in use among microscopists is the thousandth part of a millimetre. About 70 or 80 of 

 the cubic microns would fit into one blood corpuscle. 



NOTE ADDED IN PRESS. 



Add the following footnote on p. 567 : — 



See Tables, pp. 595-597, and a Diagram, p. 598, of the three series of double lines in the spectrum of 

 one of the light monad elements. The spectrum selected as an example is that of Sodium, and the spectra 

 of the others, viz. Lithium, Potassium, Ruthenium, and Caesium, are of the same character. 



