ON  PYRODEXTRESTE. 
237 
When  it  is  mixed  with  only  very  small  quantities  of  dextrine, 
it  may  be  obtained  pure  by  precipitating  it  with  alcohol  from  its 
concentrated  aqueous  solution. 
When  it  contains  a  more  considerable  quantity  of  foreign 
matters,-  it  is  precipitated  from  its  aqueous  solution  by  baryta 
water.  The  dextrine  is  not  precipitated  by  this  reagent,  whilst 
pyrodextrine  is  deposited  in  the  state  of  basic  salt.  It  is  suffi- 
cient, for  the  precipitation  to  be  complete,  to  add  to  the  liquors 
about  10  per  cent,  of  alcohol. 
When  pyrodextrine  forms  only  a  very  small  proportion  of  the 
substances  from  which  it  has  to  be  extracted,  in  the  burnt  crust 
of  bread,  for  instance,  we  may,  by  means  of  malt  or  sulphuric 
acid,  convert  into  sugar  the  starch,  soluble  starch  and  dextrine 
which  this  matter  contains  in  abundance,  destroy  the  sugar 
formed  by  fermentation,  and  extract  the  pyrodextrine,  by  means 
of  baryta,  from  the  residue  of  all  these  operations. 
Whatever  substance  may  have  furnished  the  compound  of 
baryta  and  pyrodextrine,  the  latter  may  always  be  extracted 
from  it  in  the  pure  state. 
To  arrive  at  this  result,  the  well-washed  precipitate  is  treated 
with  sulphuric  acid  in  proper  quantity,  and  the  excess  of  acid  is 
saturated  with  carbonate  of  baryta  ;  we  filter,  concentrate  the 
liquor,  and  precipitate  with  alcohol.  Pyrodextrine  is  deposited 
at  the  bottom  of  the  flask,  under  the  appearance  of  a  black  and 
thick  syrup  ;  this  syrup  is  redissolved  in  water  and  evaporated 
to  dryness. 
The  pyrodextrine  obtained  by  this  process  is  a  solid,  black  and 
brittle  substance ;  brilliant  and  elastic,  like  gum,  when  it  has 
not  been  completely  dried.  It  is  insipid  and  odorless,  unaltera- 
ble in  the  air,  insoluble  in  concentrated  alcohol,  almost  soluble 
in  weak  alcohol,  and  insoluble  in  ether. 
Water,  on  the  contrary,  dissolves  it  with  facility  ;  the  solution 
is  gummy  and  adhesive ;  it  has  a  peculiar  brown  color.  This 
color  is  quite  distinct  from  that  of  the  different  caramels ; 
it  is  duller.  This  coloring  powder  of  pyrodextrine  is  about  three 
times  as  great  as  that  of  caramelane,  but  it  is  inferior  to  that  of 
caramelene. 
Pyrodextrine  resists  the  action  of  heat  much  better  than 
sugar.  It  does  not  color  purple  red  the  aqueous  solution  of 
iodine,  as  dextrine  does. 
