Velocities  of  the  Ions  of  Alkali  Salt  Vapours.  791 
conduct  equally  well  if  the  amount  of  each  one  taken  is  pro- 
portional to  its  molecular  weight.  The  temperature  above 
which  this  result  holds  good  was  nearly  the  same  for  every 
salt.  Above  this  temperature  the  current  was  independent 
of  the  temperature  and  of  the  P.D.  used,  provided  the  latter 
was  sufficiently  great*  Since  the  lowest  temperature  at  which 
this  maximum  current  could  be  obtained  was  the  same  for 
all  alkali  salts,  it  follows  that  at  such  high  temperatures  the 
salts  tried  all  gave  ions  having  the  same  velocity;  for  if  some 
salts  had  given  ions  having  a  smaller  velocity  than  others, 
then  a  higher  temperature  and  larger  P.D.  would  have  been 
required  to  obtain  the  maximum  current  with  these  salts. 
The  writer  has  recently,  in  collaboration  with  Mr.  E.  Gold, 
B.A.,  carried  out  a  series  of  experiments  on  the  conductivity 
of  flames  containing  salt  vapours  for  rapidly  alternating- 
currents.  We  have  found  that  the  conductivity  for  rapidly 
alternating  currents  varies  as  the  square  root  of  the  corres- 
ponding conductivity  for  steady  currents.  It  is  shown  in  the 
paper  *,  that  this  result  can  be  explained  by  supposing  that  all 
the  salts  tried  give  ions  having  equal  velocities  in  the  flame. 
In  1903  Prof.  Moreau  published  an  account  (Annates  de 
Chimie  et  de  Physique,  Sept.  1903)  of  some  measurements 
of  the  velocites  of  the  ions  of  salt  vapours  in  flames  by 
a  method  which  was  described  by  the  writer  in  the  paper 
referred  to  above  in  1899.  Prof.  Moreau  found,  in  good 
agreement  with  the  earlier  work,  that  all  the  alkali  salts  tried 
gave  positive  ions  having  a  velocity  of  about  80  cms.  per  sec. 
For  the  negative  ions  of  salts  of  potassium  and  sodium,  how- 
ever, Prof.  Moreau  obtained  results  which  do  not  agree  with 
those  obtained  by  Dr.  Marx  and  the  writer.  Prof.  Moreau's 
results  for  sodium  and  potassium  salts  are  as  follows  : — 
With  equal  molecular  concentrations  the  velocity  of  the 
negative  ions  is  independent  of  the  nature  of  the  acid  radical 
of  the  salt.  It  varies  with  the  metal  in  the  inverse  ratio  of 
the  square  root  of  the  atomic  weight.  For  a  particular  salt 
vapour  the  velocity  of  the  negative  ion  increases  as  the 
concentration  of  the  vapour  diminishes.  Prof.  Moreau  gives 
the  following  numbers  : — 
Molecular  Concentration  of  Solu- 
tion sprayed  into  the  Flame  . . . 
Velocity   of    Negative    Ions    in  j  K 
cms.  per  sec ]  Na    ... 
Thus  with  large  concentrations 
have  larger  velocities  than  the  potassium  ion 
*  Phil.  Mag.  April  1906. 
i      L      i 
4          16 
1          1 
64         256* 
060      785      995 
S00    1040    1280 
1180      1320 
i  the  sodium  ions  seem   to 
