ON  A  PROCESS  OF  FRACTIONAL  CONDENSATION.  451 
tuck,*  who  made  an  extended  investigation  of  the  petroleum 
from  Sehnde,  near  Hanover,  also  with  the  use  of  Wurtz's  bulbs, 
both  assert  in  the  most  positive  manner  the  impossibility  of 
separating  from  petroleum,  by  fractional  distillation,  products  of 
constant  boiling-point. 
Such  is  the  general  character  of  the  results  obtained  in  the 
attempts  which  have  been  made  to  separate  the  constituents  of 
such  mixtures  by  fractional  distillation. 
The  treatment  with  strong  acids,  etc.,  as  an  auxiliary  to  the 
common  method  of  fractional  distillation,  which  is  claimed  to 
have  given  good  results  in  some  cases,  is  open  to  serious  objec- 
tions in  its  application  to  mixtures  of  unknown  substances,  as 
must  be  readily  apparent.  The  further  consideration  of  this 
subject  is  reserved  for  another  occasion,  when  I  shall  submit  the 
results  which  I  have  obtained  by  my  process  in  the  study  of 
mixtures  almost  identical  with  some  of  those  in  the  investigation 
of  which  the  acid  process  has  been  employed.  I  shall  then  be 
able  to  show  that  the  results  obtained  by  that  process  are,  to  a 
considerable  extent,  inaccurate  and  by  no  means  exhaustive ; 
and  that  it  is  still  of  the  highest  importance  to  have  a  process 
which  shall  be  generally  applicable  in  all  such  cases,  without  re- 
sort to  any  harsh  or  uncertain  treatment. 
With  regard  to  the  value  of  constancy  of  boiling-point  above 
referred  to,  as  a  test  of  purity  of  a  liquid  substance,  I  may  here 
say  that,  without  scarcely  lessening  the  importance  of  obtaining 
constancy  of  boiling-point,  before  resorting  to  harsher  treatment, 
in  the  study  of  mixtures  of  unknown  substances,  I  think  I  shall 
be  able  to  show,  on  another  occasion,  that  this  property  is  not 
necessarily  indicative  of  so  high  a  degree  of  purity  as  has  gen- 
erally been  supposed ;  and  that  a  body  may  have  a  constant 
boiling-point,  and  yet  contain  enough  of  a  foreign  substance  to 
*  Annalen  der  Chemie  und  Pharmacie,  cxiii.  169,  says  as  follows  :  "  Mit 
den  5°  zu  5°  aufgesaramelten  Destillaten  wurde  die  fractionirte  Destina- 
tion weider  von  Neuem  vorgenommcn,  aber  nachdem  diese  Operation 
sieben  Wochen  mit  etwas  50  Pfund  Steinol  fortgesetzt  worden  war,  erhielt 
ich  doch  kein  Product  von  irgend  constantem  Siedepunkt.  Nach  diesen 
Versuchen  halte  ich  es  fiir  Unmoglich,  das  Steinol  durch  fractionirte 
Destillationen  allein  in  Producte  mit  constantem  Siedepunkt,  zu  scheiden." 
