[     790     ] 
LXXIII.  The  Velocities  of  the  Ions  of  Alkali  Salt  Vapours  at 
High  Temperatures.  By  Haiiold  A.  Wilsojn-,  M.A., 
D.Sc,  M.Sc,  F. M.S.,  Fellow  of  Trinity  College,  Cambridge, 
and  Professor  of  Physics,  King's  College,  London*. 
IN  a  paper  on  "  The  Electrical  Conductivity  of  Flames 
containing  Salt  Vapours,"  Phil.  Trans.  A.  vol.  192, 1899, 
the  writer  gave  an  account  o£  a  series  of  experiments  which 
included  measurements  of  the  velocities  of  the  ions  of  salt 
vapours  in  flames  and  in  hot  air.  The  method  employed  was 
to  find  the  electric  intensity  necessary  to  make  the  ions  move 
against  a  stream  of  gas  moving  with  a  known  velocity.  The 
following  table  gives  the  results  obtained : — 
(1)  Positive  ions  of  various  salts  of  Caesium,  Rubidium,  Potassium, 
Sodium,  and  Lithium  in  a  Bunsen  flame.     Velocity  62  cms.  per 
sec.  for  one  volt  per  cm. 
(2)  Negative  ions  of  same  salts  in  a  Bunsen  flame.  Velocity  1000  cms. 
per  sec. 
(3)  Positive  ions  of  salts  of  Caesium,  Rubidium,  Potassium,  Sodium, 
and  Lithium  in  air  at  1000°  C.    7 "2  cms.  per  sec. 
(4)  Positive  ions  of  salts  of  Barium,  Strontium,  and  Calcium  in  air  at 
1000°  C.     3-8  cms.  per  sec. 
(5)  Negative  ions  of  salts  of  Barium,  Strontium,  Calcium,  Caesium, 
Rubidium,  Potassium,  Sodium,  and  Lithium  in  air  at  1000°  C. 
26  cms.  per  sec. 
In  1900  Dr.  E.  Marx  published  an  account  Ql  Ueber  den 
Potentialfall  und  die  Dissociation  in  Flammengasen,"  Annalen 
der  Physik,  1900,  no.  8)  of  a  valuable  series  of  experiments 
on  the  conductivity  of  flames,  including  determinations  of  the 
ionic  velocities  for  different  salt  vapours  in  the  flame.  His 
method  depended  on  observations  of  the  potential  gradient  in 
the  flame,  and  was  quite  different  from  that  employed  by  the 
writer.  He  obtained  for  the  velocity  of  the  negative  ions  of 
any  alkali  salt  in  the  flame  1000  cms.  per  sec.  For  the  positive 
ions  of  any  alkali  salt  in  the  flame  he  found  the  velocity  to 
be  about  200  cms.  per  sec. 
The  experiments  of  Dr.  Marx  and  the  writer  thus  agree  in 
showing  that  all  alkali  salt  vapours  in  flames  give  ions  having 
under  the  same  conditions  nearly  equal  velocities,  and  that 
the  velocity  of  the  negative  ions  is  much  greater  than  that 
of  the  positive  ions. 
In  1901  the  writer  published  a  paper  on  the  "  Electrical 
Conductivity  of  Air  and  of  Salt  Vapours"  (Phil.  Trans.  A. 
vol.  197,  1901),  in  which  it  was  shown  that,  above  1350°  C, 
and  with  an  E.M.F.  of  over  1000  volts,  all  alkali  salt  vapours 
*  Communicated  by  the  Physical  Society :  read  March  9,  1906. 
