NOTES  ON  BENZOIC  ACID  AND  SOME  BENZOATES. 
205 
and  if  the  process  is  continued  until  vapors  cease  to  rise,  the 
sublimate  generally  contains  a  variable  quantity  of  empyreu- 
matic  matter,  to  which  it  owes  its  odor,  and  which  likewise 
imparts  color  to  the  product.  The  acid  ought  to  be  free  of 
color,  and  to  obtain  it  thus  a  second  sublimation  is  often  found 
necessary.  In  order  to  avoid  this,  it  is  well  to  have  two  paper 
hoods  ready  made  to  fit  the  subliming  vessel,  and  remove  the 
first  before  it  has  been  requisite  to  raise  the  temperature.  At 
about  300Q  F.  a  quantity  of  benzoic  acid  may  thus  be  obtained, 
which  is  perfectly  white,  of  a  high  silky  lustre,  possessing  but 
little  of  the  empyreumatic  odor,  and  when  dissolved  in  an  alkali, 
affording  a  colorless  solution;  its  appearance  and  chemical  beha- 
viour prove  it  to  be  far  superior  to  the  article  usually  met  with  in 
commerce. 
If  the  sublimation  is  now  continued,  the  acid  obtained  subse- 
quently is  found  to  contain  empyreumatic  coloring  matter,  as  the 
process  proceeds  ;  it  may  be  freed  from  it  by  a  second  sublima- 
tion. This  increase  of  empyreumatic  products  in  the  sublimed 
acid  is  not  owing  merely  to  an  increase  of  the  temperature,  but 
principally  to  the  fact,  that  the  bibulous  paper  stretched  over 
the  vessel  has  become  saturated  with  the  same,  and  is  unable  to 
absorb  and  retain  any  more  at  the  temperature  to  which  it  is 
exposed.  If  during  the  process  the  paper  is  replaced  by  a  fresh 
piece,  the  acid  sublimes  as  white  as  before,  and  with  as  little 
odor  adhering  to  it.  This  empyreumatic  aroma  is  frequently 
considered  essential  for  the  medicinal  value  of  the  acid,  and 
such  careful  treatment  of  the  balsam  would  therefore  be  inad- 
missible when  the  acid  is  to  be  used  in  medicine.  But  I  believe 
that  the  product  which  has  been  obtained  by  a  slow  sublima- 
tion at  a  low  temperature,  will,  by  its  beauty  and  purity,  fully 
repay  for  the  little  trouble  incidental  with  its  collection. 
Benzoate  of  Soda. — When  benzoic  acid  is  almost  neutralized 
with  carbonate  of  soda,  a  colorless  solution  results,  which,  when 
sufficiently  concentrated,  yields,  on  cooling,  small  thin  needles  in 
starlike  groups :  if  somewhat  further  evaporated,  the  solution 
assumes  a  brownish  color,  and  may  remain  liquid  for  a  day  or 
two  without  separating  any  crystals.  If  still  farther  evaporated 
the  crystals  form  very  slowly  in  the  syrupy  liquid.  They  are 
very  long,  needle-shaped,  and  commence  to  become  opaque,  while 
