i66 
A  Piece  of  Pitchblende. 
f  Am.  Jour.  Pharm. 
\       April,  1909. 
be  due  to  phosphorescence,  but  were  in  some  way  connected  with  the 
uranium  contained  in  the  salts  ;  for  the  strange  property  was  exhibited 
also  by  the  non-fluorescent  uranium  compounds  and  metallic  uranium, 
even  after  these  substances  had  been  kept  for  a  long  time  in  the  dark. 
Uranium  rays  was  the  name  proposed  by  the  discoverer  for  these 
invisible  radiations,  but  they  have  since  been  re-christened  Becquerel 
rays,  and  they  are  now  recognized  as  a  manifestation  of  that  most 
remarkable  condition  of  matter  called  radio-activity. 
Let  us  see  by  what  means  these  Becquerel  rays  can  be  detected 
and  measured.  The  easiest  way,  perhaps,  is  that  by  which  Becquerel 
discovered  the  phenomenon.  It  consists  in  placing  the  substance 
under  examination,  a  piece  of  pitchblende  for  instance,  upon  a 
photographic  plate  wrapped  in  black  paper.  The  rays  then  penetrate 
the  latter  and  act  on  the  plate,  producing  an  impression  analogous  to 
that  of  ordinary  light.  It  is  thus  possible  to  make  radiographs  of 
objects  which  are  not  transparent  to  the  Becquerel  rays. 
Another  method  of  testing  for  radio-activity  is  to  allow  the 
substance  to  act  on  certain  crystalline  substances,  such  as  barium 
platinoeyanide  or  the  sulphide  of  zinc,  known  as  Sidot's  blende.  If 
the  radiations  are  fairly  strong,  they  are  made  manifest  by  the  gleam 
or  the  scintillations  they  excite  in  the  phosphorescent  material. 
More  delicate  and  far  better  adapted  for  the  measurement  of 
radio-activity,  is  the  electric  method.  This  depends  on  the  fact  that 
the  rays  emitted  by  radio-active  substances  have  the  power  to  ionize 
the  air  through  which  they  pass,  and  of  thus  conferring  upon  this  air 
the  property  of  discharging  electrified  bodies.  The  rate  of  discharge 
is  proportional  to  the  intensity  of  the  radio-activity.  The  instrument 
commonly  used  for  this  purpose  is  the  gold-leaf  electroscope,  for  the 
fall  of  the  gold  leaf  can  easily  be  observed  and  measured. 
It  was  by  the  aid  of  this  electric  method  that  one  of  the  most 
successful  investigators  of  our  time,  Mme.  Sklodowska  Curie, 
undertook,  in  1898,  to  make  a  systematic  examination  of  a  great 
number  of  substances,  including  both  elements  and  compounds,  for 
radio-activity.  She  discovered  that  thorium  is  radio-active  in  about 
the  same  degree  as  uranium,  and  that  some  of  the  uranium  and 
thorium  minerals  are  several  times  as  radio-active  as  the  two  metals 
are  in  the  free  state.  Further  inquiries  showed  that  the  radio-activity 
of  uranium  is  not  affected  by  the  combination  of  the  element  with 
other  elements,  so  that  it  must  be  regarded  as  an  atomic  property. 
The  only  way  then  that  the  excess  of  radio-activity  of  the  uranium 
