﻿NATURAL 
  RUBBER 
  — 
  COOK 
  381 
  

  

  thumb 
  and 
  first 
  finger. 
  Only 
  a 
  small 
  drop 
  of 
  latex 
  is 
  needed 
  — 
  not 
  

   enough 
  to 
  spread 
  beyond 
  the 
  opposable 
  surfaces 
  of 
  thumb 
  and 
  finger 
  

   that 
  come 
  in 
  contact, 
  where 
  the 
  skin 
  is 
  thick 
  and 
  resistant. 
  In 
  han- 
  

   dling 
  latex 
  from 
  any 
  unknown 
  plant, 
  caution 
  is 
  advised 
  not 
  to 
  permit 
  

   the 
  latex 
  to 
  touch 
  any 
  thin 
  skin, 
  as 
  between 
  the 
  fingers, 
  where 
  juices 
  

   of 
  poisonous 
  plants 
  may 
  cause 
  painful 
  injuries. 
  The 
  eyes, 
  of 
  course, 
  

   as 
  well 
  as 
  mucous 
  membranes, 
  are 
  to 
  be 
  especially 
  guarded. 
  Many 
  

   latex 
  plants, 
  especially 
  in 
  the 
  dogbane 
  and 
  spurge 
  families, 
  have 
  

   poisonous 
  properties. 
  Some 
  of 
  the 
  fleshy 
  euphorbias 
  of 
  Africa 
  and 
  

   Madagascar, 
  in 
  appearance 
  much 
  like 
  cacti, 
  contain 
  acrid 
  poisons. 
  

   The 
  rubber 
  reactions 
  can 
  be 
  learned 
  without 
  harm, 
  however, 
  if 
  the 
  

   latex 
  is 
  handled 
  carefully 
  and 
  all 
  traces 
  of 
  it 
  are 
  removed 
  by 
  dry 
  rub- 
  

   bing 
  after 
  the 
  test. 
  Native 
  information 
  regarding 
  any 
  plants 
  that 
  are 
  

   considered 
  caustic 
  or 
  poisonous 
  should 
  be 
  sought, 
  although 
  many 
  plants 
  

   that 
  are 
  feared 
  by 
  natives 
  are 
  in 
  reality 
  harmless. 
  Poisonous 
  plants 
  

   are 
  relatively 
  few 
  in 
  the 
  Tropics, 
  compared 
  with 
  our 
  poison 
  ivy 
  and 
  

   poison 
  sumac 
  so 
  widely 
  encountered 
  in 
  the 
  United 
  States, 
  or 
  with 
  

   manchineel 
  in 
  southern 
  Florida. 
  Not 
  only 
  our 
  woodlands 
  are 
  infested 
  

   with 
  poison 
  ivy, 
  but 
  villages 
  and 
  cities. 
  Precautions 
  against 
  poison 
  ivy 
  

   are 
  likely 
  to 
  be 
  effective 
  with 
  other 
  poisonous 
  plants. 
  

  

  To 
  make 
  the 
  rubber 
  assay, 
  the 
  little 
  drop 
  of 
  latex 
  is 
  pressed 
  between 
  

   the 
  thumb 
  and 
  finger, 
  which 
  then 
  are 
  separated, 
  exposing 
  the 
  surface 
  

   films 
  of 
  latex 
  to 
  the 
  air. 
  After 
  a 
  few 
  exposures, 
  fine 
  threads 
  of 
  

   elastic 
  rubber 
  may 
  be 
  seen 
  stretching 
  between 
  the 
  moist 
  surfaces, 
  

   and 
  minute 
  curds 
  of 
  coagulated 
  rubber 
  are 
  formed. 
  The 
  quality 
  of 
  

   the 
  rubber 
  may 
  be 
  judged 
  from 
  the 
  length, 
  toughness, 
  and 
  elasticity 
  

   of 
  the 
  threads 
  that 
  appear, 
  and 
  by 
  the 
  formation 
  of 
  a 
  minute 
  roll 
  

   or 
  spindle 
  of 
  rubber 
  that 
  separates 
  cleanly 
  from 
  the 
  skin 
  when 
  the 
  

   thumb 
  and 
  finger 
  are 
  rubbed 
  together. 
  If 
  of 
  good 
  quality, 
  the 
  minute 
  

   sample 
  is 
  not 
  adhesive 
  and 
  shows 
  its 
  elasticity 
  and 
  toughness 
  on 
  

   being 
  pulled. 
  Samples 
  that 
  are 
  not 
  tough 
  are 
  also 
  less 
  elastic 
  and 
  

   soon 
  become 
  sticky 
  on 
  the 
  surface, 
  while 
  samples 
  of 
  good 
  rubber 
  

   remain 
  unchanged. 
  Samples 
  of 
  poor 
  quality, 
  even 
  though 
  they 
  show 
  

   favorable 
  reactions 
  at 
  first, 
  soon 
  deteriorate, 
  sometimes 
  within 
  a 
  few 
  

   minutes. 
  

  

  When 
  a 
  flow 
  of 
  latex 
  is 
  encountered 
  in 
  an 
  unknown 
  plant 
  the 
  

   question 
  naturally 
  arises 
  whether 
  it 
  contains 
  rubber 
  that 
  would 
  be 
  

   of 
  value 
  if 
  commercial 
  quantities 
  were 
  obtainable. 
  On 
  this 
  question, 
  

   "Is 
  it 
  rubber?" 
  the 
  thumb 
  and 
  finger 
  may 
  give 
  in 
  a 
  few 
  minutes 
  a 
  

   fair 
  judgment. 
  After 
  a 
  tree 
  or 
  plant 
  is 
  seen 
  to 
  contain 
  rubber, 
  the 
  

   finder 
  may 
  be 
  interested 
  to 
  learn 
  its 
  native 
  name 
  and 
  bring 
  home 
  

   samples 
  of 
  the 
  leaves, 
  flowers, 
  and 
  fruits, 
  so 
  that 
  the 
  species 
  may 
  be 
  

   identified 
  and 
  studied 
  further. 
  Although 
  the 
  finding 
  of 
  large 
  re- 
  

   sources 
  of 
  wild 
  rubber 
  in 
  latex 
  trees 
  or 
  plants 
  that 
  are 
  abundant 
  in 
  

  

  