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  tained, 
  and 
  that 
  entomologists 
  and 
  horticulturists 
  making 
  field 
  experi- 
  

   ments 
  generally 
  use 
  the 
  gravity 
  test 
  only. 
  He 
  points 
  out 
  that 
  the 
  

   matter 
  of 
  real 
  importance 
  to 
  the 
  agriculturist 
  is 
  the 
  more 
  or 
  less 
  

   correct 
  determination 
  of 
  those 
  constituents 
  of 
  the 
  spray 
  which 
  are 
  

   really 
  insecticidal, 
  and 
  that 
  mere 
  gravity 
  is 
  of 
  little 
  or 
  no 
  value 
  for 
  this 
  

   purpose. 
  Shafer 
  has 
  shown 
  that 
  with 
  scale-insects 
  the 
  calcium 
  poly- 
  

   sulphides 
  present 
  in 
  the 
  solution 
  soften 
  the 
  so-called 
  wax 
  about 
  the 
  

   margin 
  of 
  the 
  insect 
  and 
  on 
  drying 
  cause 
  it 
  to 
  stick 
  to 
  the 
  plant 
  ; 
  

   in 
  experiments 
  made, 
  the 
  insects 
  were 
  stuck 
  down 
  tightly 
  enough 
  to 
  

   cause 
  also 
  the 
  death 
  of 
  the 
  young 
  by 
  seahng 
  them 
  under 
  the 
  scale- 
  

   cover 
  of 
  the 
  mother. 
  Shafer's 
  work 
  strongly 
  indicated 
  that 
  perhaps 
  

   the 
  principal 
  insecticidal 
  effect 
  upon 
  scale-insects 
  is 
  its 
  great 
  power 
  

   of 
  absorbing 
  oxygen, 
  thus 
  causing 
  the 
  treated 
  insects 
  to 
  suffer 
  from 
  

   an 
  insufficient 
  supply 
  of 
  this 
  element. 
  Shafer 
  further 
  showed 
  that, 
  

   contrary 
  to 
  frequently 
  expressed 
  opinion, 
  sulphur 
  dioxide 
  is 
  not 
  

   produced 
  in 
  appreciable 
  amounts 
  from 
  sulphur 
  deposited 
  from 
  the 
  

   spray, 
  except 
  at 
  temperatures 
  much 
  higher 
  than 
  those 
  generally 
  

   prevaihng 
  in 
  orchards, 
  and 
  he 
  does 
  not 
  think 
  that 
  this 
  evolution 
  of 
  

   sulphur 
  dioxide 
  can 
  be 
  regarded 
  as 
  having 
  any 
  real 
  importance 
  in 
  

   increasing 
  the 
  insecticidal 
  value 
  of 
  the 
  spray. 
  

  

  The 
  author's 
  o\\ti 
  work 
  and 
  that 
  of 
  his 
  colleagues 
  clearly 
  indicate 
  

   that 
  the 
  principal 
  insecticidal 
  constituents 
  are 
  the 
  calcium 
  poly- 
  

   sulphides. 
  Experiments 
  with 
  San 
  Jose 
  scale 
  have 
  shown 
  that 
  calcium 
  

   thiosulphate 
  has 
  Httle 
  or 
  no 
  insecticidal 
  properties. 
  Shafer 
  and 
  

   Wellington 
  have 
  arrived 
  at 
  similar 
  conclusions. 
  It 
  has 
  long 
  been 
  

   known 
  that 
  certain 
  insects 
  are 
  killed 
  to 
  some 
  extent 
  by 
  dry 
  sulphur 
  

   alone, 
  and 
  for 
  more 
  than 
  a 
  quarter 
  of 
  a 
  century 
  dusting 
  with 
  dry 
  

   sulphur 
  has 
  been 
  used 
  in 
  California 
  against 
  the 
  almond 
  red 
  spider. 
  

   Authorities 
  are 
  quoted 
  to 
  show 
  that 
  there 
  is 
  considerable 
  justification 
  

   for 
  faith 
  in 
  this 
  application 
  of 
  dry 
  sulphur. 
  It 
  has 
  also 
  been 
  suggested 
  

   that 
  hydrogen 
  sulphide, 
  which 
  is 
  poisonous 
  to 
  insects, 
  may 
  be 
  hberated 
  

   from 
  lime-sulphur, 
  but 
  experiments 
  carried 
  out 
  in 
  the 
  laboratory 
  

   showed 
  that 
  the 
  quantity 
  produced 
  is 
  so 
  exceedingly 
  small 
  that 
  it 
  

   can 
  hardly 
  be 
  of 
  any 
  consequence. 
  

  

  The 
  insecticidal 
  properties 
  of 
  hme-sulphur 
  are 
  thus 
  considered 
  to 
  

   be 
  due 
  to 
  : 
  — 
  (1) 
  Its 
  power 
  of 
  absorbing 
  oxygen 
  ; 
  (2) 
  its 
  abihty 
  to 
  

   soften 
  the 
  newly 
  secreted 
  wax 
  on 
  the 
  margins 
  of 
  scale-insects 
  ; 
  and 
  

   (3) 
  the 
  amount 
  of 
  free 
  sulphur 
  formed 
  in 
  its 
  decomposition. 
  

  

  The 
  combination 
  of 
  oxygen 
  with 
  the 
  moist 
  polysulphides 
  is 
  very 
  

   rapid, 
  but 
  combination 
  with 
  the 
  thiosulphates 
  is 
  very 
  slow, 
  too 
  slow 
  

   in 
  the 
  author's 
  opinion 
  to 
  make 
  this 
  compound 
  of 
  any 
  consequence 
  

   as 
  an 
  insecticide, 
  and 
  experiments 
  made 
  at 
  the 
  Station 
  indicate 
  that 
  

   calcium 
  sulphite 
  has 
  practically 
  no 
  insecticidal 
  effect 
  whatever 
  upon 
  

   the 
  San 
  Jose 
  scale. 
  The 
  author 
  suggests 
  that 
  what 
  he 
  thinks 
  might 
  

   be 
  expressed 
  as 
  " 
  oxygen 
  number," 
  that 
  is 
  to 
  say, 
  the 
  number 
  of 
  

   grams 
  of 
  oxygen 
  absorbed 
  by 
  100 
  grams 
  of 
  Hme-sulphur, 
  would 
  be 
  

   of 
  great 
  value 
  to 
  agriculturists 
  as 
  a 
  standard 
  of 
  efficiency. 
  He 
  also 
  

   suggests 
  that 
  a 
  number, 
  which 
  might 
  be 
  called 
  " 
  available 
  sulphur 
  

   number," 
  should 
  be 
  used, 
  this 
  term 
  meaning 
  sulphur 
  deposited 
  

   expressed 
  as 
  a 
  percentage 
  of 
  the 
  original 
  hme-sulphur 
  solution. 
  

   He 
  says 
  that 
  no 
  chemical 
  explanation 
  can 
  be 
  given 
  of 
  the 
  undoubted 
  

   softening 
  effect 
  of 
  polysulphides 
  upon 
  the 
  waxy 
  secretion. 
  

  

  A 
  very 
  full 
  bibhography 
  of 
  the 
  subject 
  is 
  given. 
  

  

  (C67) 
  B 
  

  

  