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244  SUBLIMING  TEMPERATURES  OF  POISONS. 
practical  purpose,  at  the  same  time  that  I  retained  the  advan- 
tage of  the  glass  disk.  The  simple  apparatus  which  I  devised  for 
this  purpose,  and  which  I  found  to  admit  of  ready  application, 
and  to  yield  very  satisfactory  results,  consists  of  a  plate  of 
copper,  with  a  hollow  nipple  of  the  same  metal  worked  into  the 
centre  of  it,  without  the  use  of  solder.  This  nipple  receives  the 
narrow  bulb  of  a  thermometer  indicating  all  temperatures 
np  to  700°  F;  The  thermometer  is  supported  by  a  perforated 
cork  fitting  a  ring  of  a  retort-stand.  The  apparatus  is  used  as 
follows: — A  minute  quantity  of  the  poison  or  other  substance  to 
be  examined  (say  the  TJ ^  grain)  is  placed  on  a  small  fragment 
of  microscopic  glass,  and  this  on  a  part  of  the  copper  plate, 
between  the  centre  and  circumference.  It  is  then  surrounded 
with  a  glass  ring,  on  which  a  clean  disk,  previously  dried  in  the 
flame  of  the  spirit-lamp,  is  placed.  The  flame  of  the  spirit-lamp 
is  then  steadily  applied  to  some  point  of  the  copper  plate  equi- 
distant from  the  object  and  the  thermometer.  The  instrument 
must  be  placed  in  a  good  side-light,  and  the  eye  of  the  observer 
should  be  at  such  a  level  as  to  see  with  equal  ease  the  object 
itself,  the  surface  of  the  glass  disk,  and  the  scale  of  the  ther- 
mometer. To  get  good  results  with  this  simple  instrument,  the 
flame  of  the  lamp  should  be  so  applied  that  the  temperature  may 
rise  steadily  and  rather  slowly,  so  that  each  observation  made 
on  the  substance  under  examination  or  on  the  superimposed 
disk,  may  coincide  very  nearly  with  temperature  indicated.  But 
as  the  heat  imparted  to  the  mercury  in  the  bulb  of  the  thermom- 
eter does  not.  immediately  travel  to  the  metal  in  the  upper  part 
of  the  tube,  it  is  necessary  to  withdraw  the  lamp  directly  the 
change  which  we  are  looking  for  occurs,  either  in  the  substance 
under  examination  or  on  the  disk  of  glass.  The  thermometer 
will  then  be  observed  to  rise  a  few  degrees,  and  the  highest  point 
to  which  it  attains  should  be  set  down  as  the  temperature  (ap- 
proximately) at  which  the  change  in  question  was  perceived. 
From  this  description  it  will  be  seen  at  once  that  this  method 
is  not  susceptible  of  extreme  accuracy.  When  we  speak  of  any 
particular  temperature,  as  that  at  which  a  certain  change  (as 
melting  or  subliming)  takes  place,  we  mean  the  temperature  at 
which7  using  care  and  caution,  we  first  perceived  a  change  of 
