the  a  Rays  from  Radiothorium.  795 
A  second  method  of  determining  the  activity  was  to 
compare  the  7  rays  of  the  radiothorium  with  those  of  a  large 
amount  of  commercial  thorium.  A  quantitative  comparison 
was  kindly  made  by  Mr.  Eve,  working  at  the  same  laboratory, 
by  means  of  a  sensitive  electroscope  which  was  covered  with 
a  sufficient  thickness  of  lead  to  absorb  all  the  /9  rays.  Mr. 
Eve  compared  1  kg.  of  pure  thorium  nitrate  with  10*9  mg. 
of  a  radiothorium  preparation,  which  was  formerly  measured 
by  the  writer  by  an  indirect  method  and  supposed  to  be 
about  2J  times  stronger  than  the  above-mentioned  2*7  mg. 
It  was  now  found  that  both  preparations  were  of  the  same 
order,  the  10'9  mg.  being  about  130,000  times  as  strong  as 
thorium,  when  comparing  weight  by  weight. 
These  results  led  me  to  make  a  more  careful  investigation 
to  see  whether  radiothorium  was  slowly  losing  its  activity. 
Two  thin  films  of  radiothorium,  each  about  j^  mg.,  were 
obtained  on  two  watch-glasses  and  their  activity  measured 
from  time  to  time.  A  slight  decrease  was  noticed  after 
some  months,  but  no  definite  statement  can  yet  be  made,  as  a 
very  thin  film  of  radium,  measured  under  similar  conditions, 
showed  even  a  greater  decrease.  I  have  recently  prepared 
another  sample  of  radiothorium,  placed  it  in  a  watch-glass, 
and  covered  it  with  a  thin  sheet  of  mica,  so  that  it  is  perfectly 
airtight.  Under  similar  conditions,  Eve  has  shown  that 
radium  does  not  appreciably  decrease  in  activity.  It  is 
hoped,  in  this  way,  that  it  may  be  possible  to  settle  definitely 
whether  radiothorium  loses  its  activity  at  a  measurable  rate. 
Even  if  we  shall  find  a  slow  decrease  of  the  activity  of  radio- 
thorium with  the  time,  it  is  certain  that  we  are  dealing  with 
a  very  slowly  changing  product  entirely  different  from 
thorium-X,  which  is  known  to  decay  to  half  value  in 
probably  less  than  four  days. 
Previous  experiments  indicated  that  the  a  ray  activity  of 
thorium  in  equilibrium  is  due  to  four  different  products  : 
radiothorium,  thorium-X,  emanation,  and  thorium  B, 
while  apparently  thorium  itself,  and  certainly  thorium  A,  are 
"  rayless."  In  order  to  begin  the  investigation  with  a  single 
active  product,  it  was  found  very  convenient  to  obtain  an 
active  deposit  of  thorium  B  on  a  thin  copper  wire  by  exposing 
the  negatively  charged  wire  to  the  thorium  emanation,  after 
the  method  used  by  Rutherford  for  radium.  In  the  case  of 
thorium,  it  is  not  necessary  to  use  the  active  source  in  solu- 
tion, because  unheated  thorium  oxide  or  carbonate  emanates 
strongly  in  the  solid  state.  The  10  mg.  of  radiothorium  were 
placed  on  the  bottom  of  a  small  brass  cylinder.     A  thin 
3F  2 
