578 
Analysis  of  Essential  Oils. 
{Am.  Jour.  Pbarm. 
X  December,  1399. 
that  is  necessary  is  to  turn  the  upper  handle  till  the  liquid  drops 
into  the  next  receiver.  The  only  piece  of  this  apparatus  that  has 
to  be  moved  during  the  operation  is  the  glass  rod  in  the  cork,  which 
always  will  turn  easy  and  hardly  ever  leak. 
To  regulate  the  vacuum  if  the  water  pressure  should  vary,  a  tee 
can  be  connected  with  h,  one  outlet  of  which  leads  to  the  pump 
and  the  other  one,  by  means  of  a  thick  rubber  tube,  to  a  filter  flask 
with  lateral  tube,  which  is  also  mounted  with  a  rubber  tube;  by 
squeezing  both  rubber  tubes  more  or  less  with  adjustable  pinch- 
cocks,  any  constant  air  pressure  within  the  range  of  efficiency  of 
the  pump  can  be  produced. 
Due  attention  should  also  be  given  to  the  correctness  of  the 
thermometers,  which  often  are  all  but  accurate  when  bought  from 
the  dealer,  and  which,  moreover,  will  undergo  changes  of  the  scale 
when  used  for  high  temperatures.  They  should,  therefore,  often 
be  controlled  by  determining  melting  and  boiling  points  of  pure 
substances  for  which  these  figures  are  accurately  known.  The  most 
suitable  kinds  are  those  made  from  Jena  glass,  recognizable  by  a 
lengthwise  red  line  on  the  back  of  the  stem ;  for  all  ordinary  pur- 
poses they  can  be  considered  as  correct  and  unvarying.  If  ordinary 
thermometers  with  long  stems  are  used,  the  correction  for  the  part 
not  immersed  in  the  vapors  should  not  be  neglected,  at  least  for 
stating  the  boiling  points  of  pure  substances;  such  a  correction 
becomes  unnecessary  by  the  use  of  short  thermometers  the  scale 
of  which  begins  at  6o°  or  iOO°,  and  can  be  immersed  entirely  into 
the  vapors. 
In  order  to  accelerate  the  separation  of  liquids  of  different  boiling 
points,  a  great  number  of  dephlegmators  have  been  constructed, 
many  of  which  are  quite  serviceable,  but  sometimes  apt  to  break 
or  difficult  to  clean.  Generally,  I  prefer  a  simple  tube  {Fig,  4), 
about  yA  inch  wide,  longer  for  low-boiling  and  shorter  for  high- 
boiling  substances ;  near  the  upper  end  it  has  a  lateral  discharge- 
tube,  near  the  lower  end  it  is  narrower,  so  as  to  offer  a  rest  for  some 
pieces  of  broken  glass;  upon  these  smaller  fragments  of  glass,  or 
short  pieces  of  thin  glass,  tubes  are  filled,  nearly  up  to  the  lateral 
overflow.  For  high-boiling  liquids  ordinary  fractionation  flasks  are 
used,  which  are  provided  with  an  offset  before  the  gas  blast ;  the 
space  between  the  latter  and  the  lateral  tube  is  again  filled  with 
broken  glass  {Fig.  5).    For  fractionation  in  the  vacuum,  when  the 
