152         ON  A  PROCESS  OF  FRACTIONAL  CONDENSATION. 
appreciably — and,  in  delicate  cases,  seriously — affect  the  deter- 
mination of  its  constitution  and  of  some  of  its  other  properties. 
But  in  no  such  case  have  I  yet  found  that  the  removal  of  the 
impurity  by  chemical  means  has  essentially  changed  the  boiling- 
point, — i.  e.,  never  to  the  extent  of  1°  C.  of  temperature.  I 
propose,  at  a  future  time,  to  study  this  question  synthetically, 
operating  with  pure  liquid  substances,  with  the  view  to  deter- 
mine, in  a  few  cases,  how  much  of  a  foreign  substance  may  be 
present, — which  would  probably  be  variable  in  different  cases, — 
without  sensibly  affecting  the  boiling-point.  A  solution  of  this 
question  would,  I  think,  be  of  considerable  practical  value  in 
some  instances.* 
Of  the  new  process. — The  chief  distinctive  feature  of  my  pro- 
cess, as  compared  with  the  common  one,  consists  in  this, — that 
the  operator  has  complete  and  easy  control  of  the  temperature 
of  the  vapors  given  off  in  distillation ;  and  consequently  can 
readily  cool  these  vapors  to  the  lowest  limit  of  temperature  which 
the  most  volatile  portion,  under  the  circumstances,  is  able  to 
bear  and  retain  its  vaporous  condition.  It  will  be  seen  at  a 
glance  that,  under  these  conditions,  the  operator  has  it  in  his 
power  to  secure  in  any  case  the  very  largest  possible  amount  of 
condensation  of  the  heavier  from  the  lighter  vapors.  The  liquids 
resulting  from  the  condensation  of  the  less  volatile  portions  of 
course  fall  back  into  the  retort,  while  the  vapors  of  the  more 
volatile  parts  continue  to  go  forward  to  a  cold  condenser,  de- 
scending in  the  opposite  direction,  from  which  the  condensed 
product  falls  into  a  special  receiver.  In  this  manner  he  is  able 
to  obtain,  in  each  successive  operation,  a  series  of  products  which 
shall  contain  the  minimum  quantity  of  the  less  volatile  constitu- 
ents, which  a  single  distillation  is  capable  of  affording. 
Of  the  common  process,  on  the  contrary,  nearly  the  reverse  of 
all  this  is  true  :  the  operator  having  no  control  whatever  ;  being 
forced  to  receive  the  vapors  at  the  temperature  which  they 
naturally  acquire  in  passing  from  the  retort,  and  laden  with 
*  Since  this  was  prepared  for  the  press  I  notice  that  late  experiments 
by  Berthelot  go  to  show  the  correctness  of  my  conception  of  the  value  of 
constancy  of  boiling-point,  as  above  stated. 
