792  Velocities  of  the  Ions  of  Alkali  Salt  Vapours, 
The  method  used  by  Prof.  Moreau  was  to  find  the  P.D., 
between  two  vertical  electrodes,  required  to  make  the  salt 
ions  move  across  from  the  bottom  of  one  electrode  to  the  top 
of  the  other.  Then,  assuming  the  electric  field  uniform  and 
knowing  the  upward  velocity  of  the  gases  between  the 
electrodes,  the  velocity  of  the  ions  was  calculated.  Now  the 
electric  field  in  a  flame  is  very  far  from  being  uniform,  but  it 
is  no  doubt  nearly  the  same  in  different  cases  when  the 
conductivity  of  the  flame  is  the  same.  Consequently,  if  we 
compare  Prof.  Moreau' s  results  for  potassium  salts  with  his 
results  for  sodium  salts,  taking  such  concentrations  that  the 
flame  has  the  same  conductivity  for  the  potassium  salts  as  for 
the  sodium  salts,  then  we  ought  to  get  the  relative  velocities 
of  the  potassium  and  sodium  salt  ions  free  from  error  due 
to  variations  of  the  potential  gradient  with  the  conductivity 
of  the  flame. 
Potassium  salts  conduct  about  four  times  as  well  as  sodium 
salts  when  the  concentration  is  small,  so  that  Prof.  Moreau's 
numbers  may  be  tabulated  thus  : — 
Concentration  of  Potassium  "till 
Salts    J  4  16  64 
Concentration    of    Sodium  "[-.11 
Salts    J"  4  Jig 
^taiiS"} **  995  1180 
TS^ta.1ssfr...^} 8o°    imo    i28° 
Thus,  comparing  velocities  with  concentrations  giving 
nearly  equal  conductivities,  the  velocities  found  by  Prof. 
Moreau  are  very  nearly  the  same  for  potassium  salts  as  for 
sodium  salts.  The  apparent  variation  of  the  velocity  with  the 
concentration  of  the  salt  vapour  is  no  doubt  due  to  the 
variation  of  the  fall  of  potential  between  the  electrodes  with 
the  conductivity,  and  not  to  any  real  variation  of  the  velocity 
with  the  concentration. 
The  mean  of  Prof.  Moreau' s  results  for  the  velocity  of  the 
negative  ions  is  about  1000  cms.  per  sec,  and  so  agrees  very 
well  with  the  earlier  results  obtained  by  Dr.  Marx  and  the 
writer. 
If  Prof.  Moreau  were  correct  in  supposing  that  the 
velocity  of  the  negative  ions  varies  inversely  as  the  square 
root  of  the  atomic  weight  of  the  metal,  it  would  follow  that 
salt  vapours  are  ionized  into  a  metal  ion  carrying  a  negative 
charge  and  a  positive  ion  the  same  for  all  salts.  Such  a 
conclusion  is  altogether  inconsistent  with  our  knowledge  of 
the  ionization  of  salts. 
