440  Acids  Contained  in  Plants.         {A^£S&,  w*™' 
The  precipitate  of  quinine  citrate  may  be  confounded  with  the 
quinine  acid  tartrate  (solubility  2-4  per  cent.)  and  the  acid  -succinate 
(solubility  1-2  per  cent.). 
(2)  Cinchonine  dissolved  in  methylic  alcohol  precipitates  malic 
acid  under  conditions  identical  to  those  in  which  quinine  precipi- 
tates citric  acid.  However,  the  solubility  of  acid  cinchonine  oxalate 
in  methylic  alcohol,  at  95 0  Gay-Lussac  and  in  the  cold,  which  is 
2-5  per  cent.,  is  higher  than  that  of  quinine  citrate;  but  the  other 
salts  of  cinchonine  are  so  soluble  that  the  precipitation  above  men- 
tioned may  be  considered  as  characteristic  of  malic  acid.  Acid 
cinchonine  tartrate  is,  in  fact,  soluble  at  20-6  per  cent.;  the  acid 
citrate,  the  acid  oxalate  and  the  acid  succinate  do  not  crystallize 
until  their  solutions  have  been  brought  to  the  state  of  syrup. 
The  tartaric,  citric,  oxalic  and  succinic  acids,  if  mixed  with  malic 
acid,  increase  in  a  striking  manner  the  solubility  of  cinchonine 
malate  in  methylic  alcohol ;  it  is  thus  that  citric  acid,  added  to 
malic  acid  in  the  proportions  of  20,  50,  100  per  cent.,  hinders  the 
tenth,  the  half  and  even  the  whole  of  the  oxalic  acid  from  crys- 
tallizing as  a  cinchonine  salt. 
(3)  To  apply  the  foregoing  reactions  to  the  extraction  of  the 
acids  of  a  vegetable  juice,  it  must  first  be  evaporated  in  vacuum 
and  redissolved  in  methylic  alcohol  as  concentrated  as  possible.  If 
the  juice  contains  potassium  bitartrate  and  free  tartaric  acid,  it  must 
be  treated  with  alcohol  and  ether  to  separate  the  tartar,  and  precipi- 
tate the  tartaric  acid  in  the  state  of  bitartrate  by  a  limited  addition 
of  potassa  to  the  same  ethero-alcoholic  liquor.  To  eliminate  the 
excess  of  potassa,  we  precipitate  all  the  acids  by  basic  lead  acetate, 
and  liberate  them  again  with  hydrogen  sulphide.  It  is  proper  to 
operate  in  the  same  manner  if  the  juices  contain  an  excessive 
quantity  of  sugars  or  foreign  matters. 
The  concentrated  acids  being  dissolved  in  methylic  alcohol,  we 
take  a  known  volume  of  the  liquid,  which  we  dilute  with  methylic 
alcohol,  so  that  the  solution  may  contain  2-5  per  cent,  of  acid,  and 
we  add  to  the  liquid  increasing  quantities  of  quinine  in  powder, 
until,  after  being  stirred  for  some  time,  it  sets  into  a  crystalline  mass. 
The  quantity  of  quinine  added  should  not  exceed  160  to  170  parts 
to  100  parts  of  the  citric  acid  supposed  to  be  present.  We  must, 
in  fact,  avoid  adding  an  excess  of  quinine,  which  would  dissolve — 
momentarily  at  least — the  acid  citrate  and  form  neutral  citrate, 
