464         ON  A  PEOCESS  OF  FRACTIONAL  CONDENSATION. 
curacy  when  to  add  the  next  fraction  to  the  retort.  By 
observing  this  systematic  course,  irregularities,  from  the  im- 
proper mixture  of  products,  may  be  avoided,  and  time  thus  econ- 
omized. 
After  a  few  series  of  fractionings, — sometimes  after  two  or 
three,  variable  in  number,  according  to  the  nature  or  complica- 
tion of  the  mixture, — it  will  be  found  that  some  of  the  fractions 
are  considerably  larger  than  others  for  the  same  range  of  tem- 
perature, indicating  approximately  the  boiling-points  of  the  sev- 
eral constituents.  But  fractions  of  constant  boiling-point,  or 
those  the  boiling-points  of  which  cannot  be  sensibly  changed  by 
further  fractional  condensation,  are  not  obtained,  as  already 
mentioned,  till  after  repeated  careful  fractioning  of  every  degree 
of  temperature.  When  fractioning  of  every  degree,  it  is  im- 
portant to  use  every  precaution  to  protect  the  thermometer  from 
external  influences,  and  to  carefully  apply  the  corrections  for 
variations  in  the  atmospheric  pressure.  This  may  even  be 
desirable  earlier ;  but  it  is  of  so  much  importance  in  the 
case  specified,  that,  if  omitted,  the  operator  would  be  liable  one 
day  to  mix  products  which  he  had  separated  the  day  previous. 
In  this  way,  certain  larger  fractions  are  obtained,  which  are 
not  susceptible  of  further  alteration  in  their  boiling-points ;  but 
there  are  yet  considerable  quantities  of  liquid  in  the  intermedi- 
ate fractions,  which  still  continue  to  change  more  or  less  in  each 
succeeding  operation.  When  the  fractions  of  constant  boiling- 
point  have  once  been  obtained,  if  it  were  not  important  to  test  for 
other  bodies  in  the  intermediate  fractions,  the  operation  might 
here  be  suspended,  provided  the  pure -products  already  obtained 
should  be  large  enough  for  the  purposes  required. 
But,  in  my  investigations,  I  have  undertaken  to  prove  the  neg- 
ative as  well  as  the  positive.  I  have  attempted  to  carry  the  pro- 
cess of  separation  so  far,  that  I  might  assert  the  absence  of  other 
bodies,  as  well  as  the  presence  of  those  obtained ;  and  this  clear- 
ing up  of  the  intermediate  fractions  has  generally  been  the  most 
tedious  part  of  the  work.  I  have  continued  to  operate  upon  these 
by  themselves,  until  they  almost  have  become  distributed  in  reg- 
ular course — no  new  bodies  appearing — among  the  fractions  of 
constant  boiling  point,  or  to  such  an  extent  that  the  intermediate 
