I 
ON  A  PROCESS  OF  FRACTIONAL  CONDENSATION.  461 
waste  in  the  former  case,  from  evaporation,  would  be  much 
greater. 
But  in  many  cases  it  will  be  found  that  highly  volatile  bodies 
are  present  only  in  very  small  proportion, — e.  g.,  in  viscid  pe- 
troleum like  Rangoon  tar,  and  in  the  products  of  distillation  of 
some  species  of  asphalt.  In  such,  cases,  the  requisite  quantity 
to  be  operated  upon,  to  obtain  the  most  volatile  constituents  in 
sufficient  quantity  for  anything  like  a  complete  study  of  their 
chemical  relations,  would  be  extremely  large, — too  large  to  be 
conducted  in  the  laboratory, — and  one  would  have  to  resort  to 
the  manufactory  for  the  first  distillation.  I  have  dwelt  at  some 
length  on  this  point,  having  experienced  the  disappointment 
which  one  feels,  after  months  of  labor,  on  finding  the  products 
insufficient  for  his  requirements,  when  the  expenditure  of  a  little 
more  time,  comparatively,  might  have  given  double  the  quantities 
obtained. 
In  the  first  series  of  fractioning,  I  generally  operate  on  succes- 
sive portions,  of  about  one  gallon  each,  of  the  crude  material,  and 
take  off  a  fraction  for  every  20°  C.  rise  of  temperature  of  the 
retort.  These  fractions  are  preserved  in  well-stoppered  bottles, 
and  each  carefully  labelled  with  the  temperatures  between  which 
it  was  obtained.  The  fractions  for  each  fresh  portion  of  the  crude 
material,  being  collected  between  the  same  limits  of  temperature, 
are  added  to  the  corresponding  products  from  the  preceding 
operations,  till  enough  of  the  crude  material  has  been  taken  to 
insure,  ultimately,  a  sufficiency  of  the  pure  products. 
In  the  commencement,  not  only  of  this  but  of  all  subsequent 
fractionings,  when  the  temperature  to  which  the  bath  should  be 
raised  is  unknown,  I  first  bring  the  liquid  in  the  retort  into  full 
ebullition,  so  that  a  steady  stream  of  liquid  shall  flow  back  from 
the  end  of  the  worm  into  the  retort.  I  then  carefully  raise  the 
temperature  of  the  bath  until  the  vapors  from  the  retort  pass 
through  the  heated  worm  so  freely  that  the  liquid,  in  condensing 
from  them,  shall  drop  with  tolerable  rapidity  into  the  cold 
receiver.  In  order  that  this  dropping  may  be  continuous,  it  is 
necessary  that  the  temperature  of  the  bath  should  rise  very  grad- 
ually as  the  more  volatile  constituents  of  the  mixture  are  taken 
off ;  this  is  easily  effected  by  carefully  regulating  the  flame  under 
the  bath. 
