ACIDS  OF  RHUBARB  STALKS,  TOMATOES,  ETC.  195 
Experiment  for  Oxalic  Acid, 
4.  To  prove  whether  it  was  really  oxalic  acid  which  existed 
in  the  salt  from  rhubarb,  six  grains  of  the  crystals  were  dis- 
solved in  a  small  portion  of  water,  and  the  acid  solution  was 
precipitated  with  lime  water,  affording  a  white  deposit  of  oxalate 
of  lime.  I  then  added  an  excess  of  acid  (from  rhubarb)  to  the 
precipitated  solution  ;  but  it  still  remained  insoluble  in  it.  This 
test  clearly  shows  the  presence  of  oxalic  acid  ;  but  whether 
there  is  not  a  mixture  of  acids,  this  test  cannot  alone  be  relied 
upon  to  prove. 
Experiment  for  detecting  the  presence  of  other  acids, 
5.  Not  being  positive  of  the  accuracy  of  the  above  test,  six 
grains  more  of  the  acid  were  taken,  and  dissolved  in  a  small 
portion  of  water;  the  solution  was  precipitated  with  lime-water, 
yielding  a  copious  white  precipitate  of  oxalate  of  lime ;  the 
precipitate,  washed  and  carefully  dried,  was  transferred  to  a 
crucible,  and  a  slow  heat  applied  for  about  an  hour,  for  the  pur- 
pose of  detecting  the  presence  of  any  other  acid ;  but  after  the 
application  of  the  heat,  and  the  oxalate  being  converted  into  a 
carbonate  without  blackening  the  precipitate,  I  consequently 
concluded  that  no  other  acids  were  present,  but  that  the  acid 
salt  existing  in  rhubarb  stalks  is  nothing  more  than  an  acid 
oxalate  of  potassa. 
The  following  experiment  shows  that  it  was  a  binoxalate  : 
Experiment  for  ascertaining  the  amount  of  potassa. 
6.  In  examining  the  crystals  of  rhubarb,  to  ascertain  the 
amount  of  potassa  contained  in  them,  I  proceeded  in  the  fol- 
lowing manner :  Ten  grains  of  the  acid  crystals  were  dissolved 
in  a  small  portion  of  water  ;  this  solution  was  then  precipitated 
with  pure  chloride  of  calcium  ;  the  precipitate,  being  oxalate  of 
lime,  was  then  carefully  collected,  washed  and  dried,  weighing 
nearly  four  and  a  quarter  grains  ;  thus  agreeing  as  nearly  as 
could  be  expected,  considering  the  small  quantity  under  exam- 
ination, with  the  atomic  proportion  of  binoxalate  of  potassa,  as 
146  :  64  ::  10  :  4.4. 
7.  To  prove  the  accuracy  of  the  above,  I  took  the  same  quan- 
tity of  acid  salt,  placed  it  upon  a  silver  plate,  and  applied  the 
heat  of  a  spirit  lamp  to  destroy  the  organic  matter.  The  charred 
