446 
Sir  William  Crookes. 
Am.  Jour.  Pharm. 
July,  1919. 
from  radiation"  was  pursued  by  Crookes  with  extraordinary  skill 
and  ingenuity  in  devising  experiments  in  various  directions,  and 
thus  he  opened  up  a  new  and  wonderful  field  for  research  in  molec- 
ular physics. 
After  considerable  study  of  this  radiometer  the  conviction  was 
established  that  the  motion  of  the  vanes  in  the  instrument  was  prob- 
ably due  to  their  bombardment  by  the  residual  molecules  of  the  high 
rarefied  air  in  the  bulb,  being  projected  from  the  sides  of  the  same 
against  the  movable  vanes.  Crookes  devised  many  beautiful  experi- 
ments in  this  connection  and  he  was  thus  led  on  to  the  study  of 
kathode  rays  and  the  bombardment  with  them  of  various  forms  of 
solid  matter  which  became  phosphorescent  under  their  impact.  In 
this  way,  Crookes  developed  his  theory  as  to  the  existence  of  radi- 
ant matter,  which  he  considered  a  fourth  form  of  matter,  showing 
properties  not  possessed  by  matter  in  solid,  liquid  or  gaseous  form. 
We  have  more  recently  come  to  speak  of  emanations  in  connec- 
tion with  radio-active  matter,  but  it  is  interesting  to  note  here  that 
Crookes  in  his  earlier  researches  paved  the  way  for  much  of  our 
present  understanding  of  radio-active  phenomena.  One  of  Crookes' 
inventions  developed  after  the  beginning  of  the  work  on  radio- 
activity was  the  "  spinthariscope  "  in  which  a  screen  of  natural  zinc 
sulphide  is  made  to  scintillate  by  the  alpha  particles  projected  from 
radium,  which  furnishes  us  one  of  the  most  beautiful  demonstra- 
tions, at  once  visible  to  the  eye,  of  radio-activity.  Crookes  made 
many  other  very  delicate  and  skillful  experimental  studies  in  this 
same  connection  of  radio-activity,  studying  the  radio-emanation 
upon  a  great  variety  of  materials. 
In  any  account  of  Crookes'  career,  we  must  not  omit  to  note 
many  of  his  speculative  addresses  and  papers,  as  for  instance  his 
interesting  paper  on  the  genesis  of  the  elements,  delivered  as  an 
address  to  the  Chemical  Section  of  the  British  Association  in  1886 
at  Birmingham.  He  also  became  more  or  less  of  a  spiritualist  and 
wrote  a  number  of  papers  dealing  with  these  phenomena,  which  are 
again  exciting  the  interest  of  a  number  of  scientific  men,  perhaps 
more  in  England  than  in  this  country.  One  of  the  most  interesting 
of  Crookes'  addresses  was  his  address  as  President  of  the  British 
Association  at  Bristol  in  1898  in  which  he  discussed  the  nitrogen 
problem  and  called  the  attention  of  the  scientific  world  to  the  rapid 
exhaustion  of  the  natural  nitrogen-containing  minerals  and  the 
