Wave-Lengths  loith  a  Modified  Apparatus.  697 
to  allow  E  to  remain  cool,  although  D,  enclosed  in  a  copper 
case,  is  hot  enough  to  keep  the  cadmium  vapour  uncondensed. 
The  lateral  tube  C  projects  from  the  case,  and  the  cadmium 
condensed  in  it  may  need  to  be  driven  back  from  time  to  time 
by  temporary  application  o£  the  flame  of  a  spirit-lamp  or 
bun  sen-burner. 
This  construction,  used  with  cadmium,  mercury,  and 
thallium,  has  so  far  answered  my  expectations  Cadmium 
tubes,  apart  from  failures  by  cracking,  are  said  often  to 
deteriorate  rapidly.  My  experience  did  not  contradict  this ; 
for  after  four  or  five  evenings'  work  the  red  radiation,  which 
at  first  had  been  very  brilliant,  was  no  longer  serviceable, 
although  the  green  did  not  seem  to  have  suffered  much. 
At  this  stage  the  tube  was  re-exhausted  and  then  appeared 
to  behave  differently,  the  red  radiation  being  much  better 
maintained.  One  must  suppose  that  something  deleterious 
had  been  emitted  and  been  pumped  away.  There  is  much 
in  the  behaviour  of  vacuum-tubes  which  at  present  defies 
explanation. 
To  excite  the  electric  discharge  a  large  Ruhmkorff ,  actuated 
by  five  small  storage-cells,  was  usually  employed.  Sometimes, 
especially  in  the  comparison  of  the  cadmium  radiations,  an 
alternate  current  was  substituted ;  but  there  was  no  per- 
ceptible difference  in  the  measurements.  In  this  case  a 
transformer  of  home  construction  was  fed  from  a  De  Meritens 
magneto  machine. 
The  radiations  from  zinc  (and  occasionally  from  cadmium) 
were  obtained  by  an  arrangement  similar  to  Fabry  and 
Perot's  "trembler"*.  The  behaviour  was  very  capricious. 
Sometimes,  even  when  actuated  by  five  secondary  cells  only, 
the  zinc  rings  were  magnificent;  but  the  deterioration  was 
usually  rapid  as  the  zinc  points  lost  their  metallic  surfaces. 
This  change  appears  to  be  independent  of  oxidation.  When 
the  current  was  from  a  dynamo  giving  about  80  volts,  the 
apparatus  was  less  troublesome,  but  even  then  required 
careful  management.  The  fineness  of  the  points  needs  to  be 
accommodated  to  the  current  employed. 
As  an  example  of  the  observations  and  calculations  there- 
from, I  will  take  a  series  of  Dec.  20,  1905,  relative  to  the 
three  radiations  from  the  cadmium  vacuum-tube.  In  this 
series  the  temperature  conditions  were  more  favourable  than 
usual. 
*  C.R.  130.  p.  400(1900). 
Phil  Mag.  S.  6.  Vol.  11.  No.  05.  May  1906.  '2  Z 
