SUBLIMING  TEMPERATURES  OF  POISONS. 
245 
form  in  the  substance,  or  a  mist  upon  the  disk.  We  may  miss 
the  precise  moment  of  the  change,  and  make  too  high  an  esti- 
.mate  of  the  temperature.  Such  errors  of  observation  have  been 
guarded  against,  in  the  case  of  the  results  presently  to  be 
detailed,  by  repeating  the  experiment  several  times  with  the 
same  substance.  Proceeding  thus,  I  have  rarely  found  occasion 
to  correct  my  first  entries  of  results  ;  and  I  believe  that  the 
method  will  be  found  to  yield  approximations  sufficiently  close 
for  every  practical  purpose,  if  we  only  adopt  the  simple  and  ob- 
vious precaution  of  not  attaching  importance  to  differences  of  a 
few  degrees  between  the  melting  or  sulfliming  points  of  two 
poisonous  substances.  If,  for  instance,  we  experiment  with  a 
substance  which  melts  at  220°,  and  assume  it  to  be  codeia,  it 
would  be  a  valid  objection  that  paramorphia  and  papaverine  melt 
at  210°.  So  also  with  aconitia  and  atropia.  A  temperature 
of  140°  would  not  justify  us  in  assuming  that  we  are  dealing 
with  aconitia,  seeing  that  atropia  melts  at  150°.  But  if  we 
suppose,  that  a  minute  quantity  of  a  white  powder  is  placed  in 
our  hands,  and  that  it  sublimes  at  210°,  we  know  that  it  cannot  be 
.  veratria,  which  melts  at  200°,  or  paramorphia  or  papaverina, 
which  undergo  the  same  change  at  210°,  or  codeia,  which  melts 
at  220°.  The  substance  belongs  to  a  wholly  different  group.  It 
might  be  muriate  of  ammonia,  which  sublimes  at  210°,  or  pow- 
dered cantharidine,  which  sublimes  at  212°,  or  even  corrosive 
sublimate,  which  sublimes  at  200°,  either  not  melting  at*  all,  or 
melting  at  a  higher  temperature  after  subliming. 
These  observations  will  have  prepared  the  way  for  a  classifica- 
tion of  poisonous  and  other  substances,  which  promises  to  be  of 
practical  diagnostic  value.  In  performing  these  experiments  in 
the  manner  indicated,  we  encounter  three  leading  sets  of  phe- 
nomena : — 
1.  Sublimation  without  change  of  form  or  color. 
2.  Sublimation  without  change  of  form  or  color,  followed  by 
melting,  with  or  without  change  of  color  and  further  sublima- 
tion. 
3.  Melting  and  change  of  color,  followed  by  sublimation. 
I  now  proceed  to  arrange  the  principal  poisons  under  these 
three  heads,  giving  the  results  obtained  with  the  apparatus  as 
above  described. 
