280 
rochleder's  proximate  analysis. 
afforded,  I  prefer  the  direct  concentration  of  a  distillate,  -which  contains 
little  volatile  ingredients,  to  cohobation.  The  concentration  is  accom- 
plished in  the  following  manner: — The  distillate  poor  in  volatile  sub- 
stances is  mixed  with  so  much  dry  common  salt  as  it  will  dissolve,  and 
the  resulting  fluid  is  subjected  to  distillation.  Generally,  after  half  the 
water  has  distilled  off,  only  pure  water  passes  over.  The  distillation  is 
then  interrupted,  the  distillate  is  again  saturated  with  common  salt,  and 
re-distilled.  By  the  repetition  of  the  distillation  as  often  as  it  appears 
requisite,  a  concentration  can  be  effected  which  affords  a  highly  concen- 
trated solution  of  the  substances  under  examination  soluble  in  water,  and 
accomplishes  a  separation  of  the  greatest  part  of  the  substances  difficultly 
soluble  in  water.  If  the  concentration  is  not  accompanied  by  the  separa- 
tion of  oily  bodies  by  this  treatment  with  common  salt,  then  only  bodies 
are  present  which  are  very  soluble  in  water.  To  obtain  these,  two  methods 
are  proposed  ;  either  the  fluid  is  saturated  with  common  salt  and  agitated 
with  ether,  Avhich,  when  it  separates,  is  removed,  and  may  be  evaporated, 
or  the  distillate  is  treated  with  chloride  of  calcium.  When  ethereal  oils 
with  high  boiling  points  are  present,  the  treatment  with  ether  is  to  be  pre- 
ferred. The  ether  evaporates  quickly  when  exposed  in  a  dish  to  the  air  ; 
and  to  prevent  the  evaporation  of  the  volatile  oil  with  it,  care  must  be 
taken  tliat  the  dish  which  contains  the  ethereal  solution  of  the  volatile 
oil  is  set  in  a  place  where  a  good  current  of  air  rapidly  conducts 
away  the  ethereal  vapor.  On  the  contrary,  when  volatile  oils  with 
rather  low  boiling  points  are  present,  the  treatment  with  chloride  of 
calcium  has  the  advantage.  Into  the  fluid  concentrated  by  distillation 
with  common  salt,  chloride  of  calcium  in  fragments  is  thrown  in  until  the 
fluid  appears  as  thick  as  syrup  ;  it  is  then  subjected  to  distillation.  The 
distillate  is  again  treated  with  chloride  of  calcium,  and  re-distilled.  By 
three  or  four  repetitions  of  these  operations  either  a  separation  of  the  dis- 
solved substances  ensues  (^sometimes  only  a  partial  one,)  or  the  concentra- 
tion has  so  far  progressed  that  the  substance  can  be  freed  from  water  by 
the  addition  of  a  large  quantity  of  fused  chloride  of  calcium,  and  distilla- 
tion, and  may  be  obtained  as  a  distillate,  which  certainly  will  be  the  case 
when  the  body  possesses  a  boiling  point  not  much  above  212*^  Fahr.  It  is 
clear  that  a  concentration  of  this  kind  is  quite  as  little  necessary  as  coho- 
bation, when  only  acid  or  basic  bodies  are  contained  in  the  distillate. 
Before  I  conclude  the  examination  of  the  distillate,  or  rather  the  volatile 
bodies  present  in  it,  I  must  here  make  some  observations  on  the  so-called 
fractional  distillation. 
Fractional  disUllation  is  recommended  for  the  separation  of  mixed  vola- 
tile substances.  As  this  operation  is  generally  performed,  it  is  certainly 
suitable  for  the  preparation  in  a  pure  state  of  the  least  volatile  bodies  in  a 
mixture  of  that  kind,  but  is  not  adapted  for  obtaining  the  more  volatile 
constituents,  because  at  the  temperature  by  which  the  mixture  boils,  many 
of  the  constituents  are  evaporated  which  do  not  boil  at  that  temperature. 
