Alkali Salt Vapours in Flames. 119 



positive ion for only a fraction of the time. The process of 

 recombination probably sets up vibrations inside the atom 

 which produce radiation, so that the radiation is emitted by 

 the atoms when neutral, or at any rate when with less than 

 their maximum positive charge. The great facility with 

 which the alkali metals lose electrons makes it possible that 

 at the very high temperature of the flame they may lose 

 more than one negative electron. The velocity of the 

 positive ions of salt vapours in flames has been deter- 

 mined by the writer and by others, so that a knowledge of 

 their coefficients of diffusion enables their charge to be calcu- 

 lated by the well-known method first used by Townsend*. 



The method I have used to get the coefficients of diffusion 

 is ver}' simple. A small bead of salt on a platinum wire is 

 put in the flame and the shape of the region surrounding the 

 bead which emits light is measured. From this and the 

 velocity of the flame gases the coefficient of diffusion can be 

 easily calculated. 



The bead of salt evaporates at a very constant rate, and 

 can be regarded as a constant point source of salt vapour in 

 the stream of flame gases. The vapour from the bead 

 diffuses into the surrounding gas, and its concentration at 

 any point can be easily calculated by the theory of diffusion. 



Take the origin at the bead and the axis of % along the 

 direction of motion of the flame gases, and let their velocity 

 be v. Consider a small rectangular parallelipiped with sides 

 ~dx, ~di/, ~dz, with its centre at the point a, y, z. Let c denote 

 the amount of salt vapour per c. c. and K its coefficient of 

 diffusion. The amount of vapour entering a side of the 

 parallelipiped of area ~fty ~ftz is 



the amount leaving through the opposite side is 



{- K (£ + ! 9 *S) +v ( c+ i^s)}* 3 * 



Hence the rate of increase of the amount inside for these two 

 sides is 



* Phil. Trans. A. vol. cxciii. p. 129 (1900). 



