794  Dr.  0.  Halm  on  some  Properties  of 
the  thorium  emanation,  and  the  characteristic  active  deposit. 
On  account  of  its  great  activity,  radiothorium  is  a  far  more 
suitable  source  than  the  feebly  active  thorium  for  the 
investigation  of  the  properties  of  its  radiations. 
In  this  paper  an  account  will  be  given  of  investigations  on 
the  following  points  : — 
1.  Range  of  the  a.  particles  in  air  from  the  active  deposit 
of  radiothorium  by  the  methods  of  scintillations  and 
by  the  electrical  method  of  Bragg  and  Kleeman. 
2.  Proof  that  the  product  called  thorium  B  is  complex,  and 
consists  of  two  distinct  a,  ray  products  which  have 
different  ranges  of  ionization  in  air. 
3.  The    magnetic    and    electrostatic    deflexion    of    the    a 
particles  from  the  active  deposit  (preliminary). 
Activity  of  Radiothorium. 
Before  discussing  the  properties  of  the  a  rays,  a  brief 
account  will  be  given  of  some  measurements  that  have  been 
made  to  determine  the  activity  of  the  preparation  of  radio- 
thorium which  had  been  used  in  these  experiments. 
The  first  method  was  the  same  as  that  used  in  the  previous 
paper.  The  emanating  power  of  a  known  weight  of  radio- 
thorium was  compared  with  that  of  a  known  weight  of 
thorium  nitrate  under,  as  far  as  possible,  similar  conditions. 
10  grs.  of  pure  thorium  nitrate  were  dissolved  in  35  c.c. 
of  water,  and  the  emanating  power  measured  by  passing  a 
constant  air-current  through  the  solution  into  a  testing 
vessel,  the  rate  of  movement  of  the  needle  of  an  electrometer 
being  a  measure  of  the  amount  of  emanation  present.     Then 
tq  mg.  of  radiothorium  was  measured  under  exactly  the  same 
conditions,  10  grs.  of  inactive  sodium  chloride  being  added 
to  have  about  the  same  concentration  of  the  solution.     The 
jq  mg.  had  been  prepared  by  successive  dilution  from  a  pre- 
paration of  about  2'1  mg.  of  radiothorium,  the  activity  of 
which  had  been  calculated  in  London  to  be  somewhat  over 
200,000.  This  time  the  strength  of  the  ^  mg.  to  the  10  grs. 
was  found  to  be  as  1*5  :  1,  showing  that  the  radiothorium 
gave  about  150,000  times  more  emanation  than  an  equal 
weight  of  thorium.  The  order  of  magnitude  of  the  active 
thorium  product  was,  therefore,  almost  the  same  as  was 
expected,  though  the  result  seems  to  show  that  the  propor- 
tionality of  the  emanating  power  to  the  weight  does  not 
remain  constant  for  a  very  large  difference  in  concentration. 
