﻿1875.] 
  

  

  Action 
  of 
  Light 
  on 
  Selenium. 
  

  

  537 
  

  

  action 
  through 
  media 
  which 
  absorb 
  all 
  the 
  more 
  chemically 
  active 
  rays 
  is 
  

   very 
  nearly 
  as 
  great 
  as 
  when 
  they 
  are 
  not 
  interposed, 
  so 
  that 
  the 
  chemical 
  

   rays 
  produce 
  very 
  little 
  effect. 
  

  

  Experiments 
  with 
  the 
  lime 
  light, 
  with 
  rock-salt, 
  alum, 
  and 
  quartz, 
  and 
  

   their 
  combinations, 
  two 
  together, 
  show 
  that 
  the 
  resistance 
  diminishes 
  at 
  

   the 
  same 
  rate 
  as 
  the 
  illumination 
  increases. 
  This 
  seems 
  to 
  show 
  that 
  the 
  

   action 
  is 
  almost 
  entirely 
  due 
  to 
  the 
  illuminating 
  power 
  of 
  the 
  light 
  falling 
  

   on 
  the 
  selenium. 
  

  

  Experiments 
  with 
  the 
  electric 
  light, 
  with 
  smoked 
  rock-salt, 
  alum, 
  and 
  a 
  

   solution 
  of 
  iodine 
  in 
  bisulphide 
  of 
  carbon 
  show 
  that 
  the 
  obscure 
  heat-rays 
  

   do 
  not 
  act 
  powerfully 
  on 
  the 
  selenium. 
  

  

  In 
  one 
  series 
  of 
  experiments 
  an 
  attempt 
  was 
  made 
  to 
  separate 
  the 
  

   instantaneous 
  effect 
  from 
  the 
  gradual 
  effect 
  of 
  the 
  light. 
  

  

  This 
  was 
  done 
  by 
  first 
  balancing 
  the 
  resistance 
  of 
  the 
  selenium 
  before 
  

   exposure 
  by 
  a 
  resistance 
  R 
  of 
  the 
  coils, 
  then 
  diminishing 
  B, 
  by 
  300, 
  400, 
  

   or 
  500 
  ohms, 
  according 
  to 
  the 
  brightness 
  of 
  the 
  light, 
  so 
  as 
  to 
  get 
  no 
  

   sudden 
  deflection 
  when 
  the 
  current 
  is 
  made 
  at 
  the 
  first 
  instant 
  of 
  expo- 
  

   sure. 
  

  

  It 
  was 
  difficult 
  to 
  determine 
  beforehand 
  by 
  estimation 
  what 
  diminution 
  

   of 
  E 
  should 
  be 
  made 
  ; 
  but 
  after 
  several 
  trials 
  it 
  was 
  quite 
  possible 
  to 
  make 
  

   the 
  sudden 
  deflection 
  very 
  small, 
  either 
  on 
  one 
  side 
  of 
  the 
  zero 
  or 
  the 
  

   other, 
  and 
  to 
  keep 
  the 
  needle 
  near 
  the 
  zero 
  by 
  continuing 
  to 
  diminish 
  the 
  

   value 
  of 
  B 
  as 
  long 
  as 
  the 
  exposure 
  lasted. 
  

  

  In 
  this 
  way 
  the 
  effects 
  of 
  exposure 
  in 
  successive 
  equal 
  intervals 
  of 
  time 
  

   can 
  be 
  measured. 
  

  

  The 
  light 
  allowed 
  to 
  pass 
  through 
  the 
  coloured 
  glasses 
  and 
  other 
  

   absorbing 
  media 
  was 
  examined 
  by 
  a 
  spectroscope, 
  and 
  it 
  was 
  found 
  that 
  

   the 
  yellowish-green 
  rays 
  were 
  among 
  the 
  most 
  active 
  in 
  altering 
  the 
  

   electrical 
  state 
  of 
  the 
  selenium. 
  

  

  A 
  series 
  of 
  experiments 
  was 
  made 
  to 
  determine 
  the 
  effect 
  of 
  light 
  from 
  

   different 
  sources. 
  

  

  A 
  Bunsen 
  burner 
  was 
  employed, 
  and 
  chloride 
  of 
  barium, 
  chloride 
  of 
  

   strontium, 
  thallium, 
  and 
  sal-ammoniac 
  were 
  introduced 
  into 
  the 
  flame. 
  

  

  The 
  effect 
  with 
  barium 
  seemed 
  to 
  be 
  less 
  than 
  with 
  strontium. 
  

  

  "With 
  sal-ammoniac 
  in 
  the 
  flame 
  the 
  effect 
  was 
  as 
  great 
  as 
  with 
  strontium 
  

   and 
  more 
  lasting. 
  

  

  With 
  thallium 
  the 
  effect 
  was 
  considerably 
  greater, 
  more 
  gradual, 
  as 
  well 
  

   as 
  far 
  more 
  lasting 
  than 
  with 
  strontium. 
  

  

  The 
  effect 
  on 
  repeating 
  an 
  experiment 
  is 
  very 
  much 
  less 
  than 
  the 
  effect 
  

   of 
  the 
  first 
  exposure 
  with 
  each 
  new 
  source 
  of 
  light. 
  

  

  Experiments 
  were 
  made 
  with 
  the 
  Bunsen 
  burner 
  alone 
  in 
  its 
  ordinary 
  

   state 
  and 
  when 
  it 
  is 
  rendered 
  luminous 
  by 
  stopping 
  the 
  air-holes. 
  

  

  Exposure 
  to 
  the 
  ordinary 
  Bunsen 
  flame 
  for 
  several 
  seconds 
  only 
  caused 
  

   a 
  slight 
  deflection 
  of 
  about 
  10 
  divisions 
  of 
  the 
  scale. 
  After 
  this 
  slight 
  

   diminution 
  of 
  resistance 
  the 
  needle 
  gradually 
  returned 
  to 
  zero, 
  and 
  was 
  

  

  