^  M'ay';Y92":}        When  Is  Poisou  Not  a  Poison?  325 
the  whole  amount  be  taken  at  a  single  dose.  Indeed,  dilution  may 
even  increase  its  activity. 
Be  it  said  that,  although  arsenic,  morphine  and  such  as  these 
act  as  poisons  upon  the  normal  man,  an  individual  may,  as  above 
stated,  accustom  his  system  to  the  drug,  so  that  enormous  doses 
may  be  taken  without  apparent  injury.  In  this  no  comparison  can 
be  made  with  the  cited  action  of  sulphuric  acid  of  which  a  drop  on 
the  skin  of  any  man  will  bite  its  way  to  the  tissue  beneath,  but  yet 
can  with  impunity  be  swallowed  when  diluted  with  water. 
We  may  likewise  pass  to  other  material  that  exercise  special 
influences,  but  where,  so  far  as  we  know,  artificial  habit-forming 
methods  have  no  part.  Rhus  toxicodendron  both  excretes  a  sub- 
stance and  carries  a  volatile  something  that  produces  violent  toxic 
action  on  some  persons,  while  to  others  it  is  as  harmless  as  bedewed 
grass.  A  waft  of  air  over  the  dew-covered  vine  may  close  the  eyes 
of  a  strong  man  exposed  to  its  air- wafted  influence,  may  cover  his 
body  with  a  most  painful  eruption,  may  drive  him  to  seek  his  phy- 
sician's aid.^  Another  man  or  a  fragile  girl  comes  next  and  with 
impunity,  with  bare  hands,  pulls  the  vine  from  its  fastenings.  The 
eyes  of  the  first  man  may  be  closed  by  the  attenuated  "poison," 
imperceptibly  attenuated  by  the  gentle  breeze,  beyond  the  chemist's 
art  to  indentify,  while  the  other  person,  bespread  with  its  juice, 
has  not  even  a  pimple  on  his  hand.  Thus,  "poison  ivy"  is,  or  is  not, 
a  poison,  as  the  individual  is  or  is  not  susceptible  to  its  influence. 
As  an  illustration,  the  writer  each  year  has  laboratory  use  for 
many  thousand  pounds  of  rhus  toxicodendron  (poison  ivy).  The 
green  leaves,  when  in  their  prime,  are  gathered  by  collectors,  who 
in  midsummer,  with  bare  hands  and  arms,  strip  the  vines,  crush  the 
green  leaves  into  sacks,  and  deliver  the  product  with  impunity. 
No  immediate,  or  after-effect,  is  noticeable.  One  young  lady  of  the 
laboratory  force  is  so  sensitive  to  the  action  of  the  drug  as  to  respond 
to  the  emanations,  although  she  be  in  a  distant  part  of  the  estab- 
lishment. To  bottle  and  label  "Rhus,"  or  otherwise  handle  a  prep- 
aration, means  to  her  typical  rhus  poisoning.  Consequently,  at 
such  time,  she  has  a  vacation,  not  being  allowed  within  the  estab- 
ishment. 
Once  we  knew  a  man  to  be  vaccinated  with  a  virus-crust  that, 
used  in  equal  amount  on  others,  produced  no  untoward  action. 
And  yet  that  man  came  near  losing  his  arm.  It  is  evident  that  not 
the  virus,  but  the  man,  or  some  undetermined  local  cause  connected 
