1 84  Views  on  Constitution  of  the  Atom.    {Am-^i,' m5Tm' 
When  the  seventh  Duke  of  Devonshire  presented  the  Cavendish 
Laboratory  to  the  University  of  Cambridge,  the  first  Professor  of 
Experimental  Physics  appointed  was  Clerk  Maxwell,  the  Newton  of 
electricity.3  In  his  great  treatise  on  "  Electricity  and  Magnetism  " 
he  made  a  notable  prophecy ;  namely,  that  the  discharge  of  electricity 
through  a  rarefied  gas  is  a  phenomenon  of  great  interest  and  im- 
portance, and,  when  better  understood,  "  will  probably  throw  great 
light  on  the  nature  of  electricity  as  well  as  on  the  nature  of  gases  and 
of  the  medium  pervading  space." 
When  Maxwell  died  he  was  succeeded  by  Lord  Rayleigh,  who 
found  the  units  of  electricity  uncertain  to  some  five  per  cent.  This 
matter  needed  prompt  attention,  for  electrical  supply  was  then  be- 
coming a  commercial  undertaking.  Rayleigh,  in  a  few  years,  made 
it  possible  to  determine  with  good  accuracy  the  volt,  the  ampere,  and 
the  ohm.  He  retired,  owing  to  his  multitudinous  duties  and  in- 
terests, and  the  University  of  Cambridge  was  somewhat  startled  to 
find  the  great  Chair  of  Physics  conferred  on  a  "  boy  "  of  twenty- 
seven,  the  illustrious  J.  J.  Thomson. 
It  would  seem  that  he  was  born  to  prove  the  accuracy  of  the  first 
half  of  Maxwell's  prophecy,  for  he  promptly  investigated  electrical 
discharges  in  gases,  and  the  timely  discoveries  of  the  cathode  rays 
and  of  Rontgen  rays  enabled  him  to  make  two  great  forward  steps, 
namely,  the  theory  of  gaseous  ionization,  and  a  clear  view  of  the 
nature  of  electrons. 
When  a  current  is  passed  from  metal  plates  through  a  highly- 
exhausted  tube  a  stream  of  particles  is  projected  from  one  plate  (the 
cathode).  It  was  known  that  these  were  readily  deflected  by  a 
magnet,  but  Sir  Joseph  Thomson  succeeded  in  deflecting  them  also 
by  an  electrostatic  field.  He  thus  showed  that  the  cathode  particles, 
or  corpuscles,  are  exceedingly  light,  with  a  mass  about  one-eighteen- 
hundredth  part  of  the  hydrogen  atom.  These  corpuscles  are  now 
usually  termed  electrons,  and  are  the  fundamental  units  or  atoms  of 
negative  electricity.  It  is  scarcely  credible  that  this  most  primary 
conception  can  ever  be  dethroned  from  a  foremost  place  in  human 
knowledge. 
Sir  Joseph  Thomson  also  gave  us  a  theory  of  the  manner  in 
which  a  gas  conducts  electricity.  If  the  molecules  were  all  un- 
charged, then  he  believed  that  the  gas  would  be  a  perfect  insulator. 
3  Maxwell  himself  termed  Ampere  the  Newton  of  electricity. 
