Am  Jour  Pharm.  i    y{ews  on  Constitution  of  the  Atom.  185 
April,  1915.       f  J  . 
The  current  of  electricity  in  a  gas  is  always  carried  by  "  ions,"  or 
charged  molecules  of  gas  which  have  gained,  or  lost,  one  or  possibly 
more  electrons.  At  low  pressures  or  high  temperatures  the  negative 
electricity  may  be  conveyed  by  electrons  instead  of  by  negative  ions; 
but  positive  electricity  has  never  been  found  detached  from  atoms, 
or  from  the  nucleus  of  atoms,  so  that  the  term  positive  electron  has 
at  present  no  concise  meaning.  Later  we  shall  see  Sir  Ernest  Ruther- 
ford's suggested  interpretation  of  it. 
It  is  necessary  to  write  a  few  words  about  the  radio-active  sub- 
stances, or  radiants,  as  I  find  it  convienent  to  call  them. 
There  are  about  thirty-seven  radiants  known,  and  they  are  divided 
into  three  great  families:  (1)  uranium- radium,  (2)  thorium,  (3) 
actinium. 
When  any  one  of  the  atoms  disintegrates  it  projects  a  funda- 
mental part  of  the  atom,  either  an  alpha  ray  or  a  beta  ray,  with  a 
high  velocity  into  the  surrounding  space.  That  which  remains  of  the 
old  atom  is  a  new  atom  of  different  physical  and  chemical  properties 
to  the  parent  atom  from  which  the  ejection  took  place. 
The  beta  particle  is  an  electron.  The  alpha  particle  is  a  helium 
atom  stripped  of  two  electrons.  As  the  alpha  particle  slows  down  it 
picks  up  two  electrons  from  some  neighboring  matter,  and  it  is  then 
an  ordinary  helium  atom,  and  emits  the  same  spectrum  as  helium. 
Radiants  may  also  emit  gamma  rays,  which  are  closely  analogous 
to  Rontgen  rays  and  are  doubtless  similar  to  light,  consisting  of  ether 
undulations,  but  with  wave-lengths  several  hundreds  of  times  shorter 
than  those  of  light. 
Much  theoretical  deduction  was  confirmed  when  C.  T.  R.  Wilson 
photographed  illuminated  ions,  mist-ladened,  marking  the  tracks  of 
electrons,  which  had  been  hurled  forth  by  a  pencil  of  Rontgen  rays 
(Fig.  1). 
As  Bragg  surmised,  the  Rontgen  rays  did  not  ionize  the  gas 
directly.  They  ejected  high-speed  electrons,  and  these  electrons, 
passing  through  the  molecules,  drove  some  low-velocity  electrons 
from  the  latter,  thus  giving  rise  to  pairs  of  ions.  The  beautiful 
photographs  of  C.  T.  R.  Wilson  showed  this  very  clearly.  They 
bring  us,  moreover,  abruptly  face  to  face  with  one  of  the  profound 
mysteries  in  physics. 
The  Rontgen  rays  are  caused  by  the  quick  stoppage  on  a  target 
of  fast-moving  electrons  in  the  Rontgen  tube.  These  rays,  like  light, 
must  spread  outwards  with  the  flux  of  energy  decreasing  by  the  law 
