-^  Tungsten   (Wolfram),   W,  Atomic  Weighty   182.6,  is  found   in  nature   as    wol-
framiic;  an  iron  and  manganese  tungstate  and  as  scheelitc  or  calcium  tun^state.
Like  molybdenum,  it  forms  a  great  number  of  compounds,  and  as  tungsttc  acid
combines  with  ba.sos  to  form  a  great  variety  of  salts.
.  _  Ux^iUuni^.  X^. --l/om*c  Weight,   236.7,  occurs  in  nature  chiefly  as    the    mineral
urantie,  or  pitch-blende,  U3O8.     It  has  many  characters  in  common  with  molyb-
^  denum  and  tungsten,  but  possesses  more  basic  properties.      The  metal  is  not  so
]X)rmanent,  and  ignites  when  heated  with  access  of  air  to  iqo°.      It  has  a  specific
gravity  of  18.4.
:^\     Three  chlorides  of  uranium  are  known,  as  follows:  the  trichloride,  UCI3,  the  tetro"
^    chloride,  UCI4,  and  the  Pentachloride,  UCU.
The  dioxide,  UO2.  ana  the  trioxide,  UO3,  are  known;  the  latter  is  basic  towards
acids,  and  forms  salts  containing  the  bivalent  radical  uranyl,  UOa.  The  uranyl
salts  arc  yellow,  and  impart  to  glass  when  fused  with  it  a  remarkable  fluorescence.
RADIO-ACTIVE    ELEMENTS.
The  fact  that  pitch-blende,  the  uranium  mineral,  gave  out  rays
which  act  upon  photographic  plates  when  the  latter  are  wrapped  in
black  paper,  cause  certain  phosphorescent  substances  to  become  illu-
minated, and  make  the  air  through  which  they  pass  a  conductor  of
electricity,  was  discovered  by  the  physicist  Becquerel.  These  prop-
erties were  found  to  be  possessed  by  all  uranium  and  thorium  minerals
as  well  as  the  metals  themselves.  Madame  Curie,  having  found  that
these  properties,  known  collectively  as  "radio-activity,"  were  possessed
by  certain  uranium  and  thorium  minerals  in  higher  degree  than
shown  by  either  of  these  metals,  was  led  to  the  conclusion  that  these
minerals  contained  one  or  more  new  elements  to  which  this  property
belonged  in  a  much  higher  degree  than  to  uranium  or  thorium.
