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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1943 
  

  

  It 
  will 
  be 
  seen 
  that 
  even 
  very 
  resistant 
  bacteria 
  are 
  quickly 
  destroyed 
  

   by 
  summer 
  sunlight, 
  but 
  that 
  even 
  the 
  more 
  sensitive 
  organisms 
  will 
  

   not 
  be 
  destroyed 
  by 
  all-day 
  exposure 
  to 
  midwinter 
  sunlight 
  in 
  the 
  

   higher 
  latitudes. 
  

  

  Figuee 
  28. 
  — 
  Seasonal 
  variation 
  in 
  solar 
  bactericidal 
  effect. 
  (3.0 
  mm. 
  zenith 
  

  

  ozone, 
  43° 
  N. 
  latitude.) 
  

  

  It 
  is 
  difficult 
  to 
  evaluate 
  the 
  full 
  significance 
  to 
  the 
  human 
  race 
  of 
  

   this 
  solar 
  bactericidal 
  action. 
  Ultraviolet 
  radiation 
  from 
  the 
  sun 
  

   and 
  desiccation 
  are 
  the 
  two 
  great 
  natural 
  agencies 
  for 
  destroying 
  bac- 
  

   terial 
  growth, 
  and 
  of 
  these 
  the 
  former 
  is 
  probably 
  the 
  more 
  im- 
  

   portant. 
  Here 
  again 
  man's 
  very 
  existence 
  must 
  depend 
  upon 
  ultra- 
  

   violet 
  radiation, 
  without 
  which 
  bacteria 
  and 
  other 
  micro-organisms 
  

   would 
  crowd 
  him 
  from 
  his 
  place 
  in 
  the 
  sun. 
  

  

  