372 
ItOCHLEDER'S  PROXIMATE  ANALYSIS. 
are  remarkably  colored  by  chlorinated  lime.  With  many  bodies  chlori- 
nated lime  produces  no  coloration.  The  chloride  of  calcium  admixed  has 
no  influence  in  these  reactions. 
14.  Bichromate  of  potash. — Bichromate  of  potash  acts  on  many  organic 
substances  at  ordinary  temperatures,  on  others  only  at  a  boiling  heat. 
Many  bodies  undergo  no  change  at  higher  temperatures  with  this  salt- 
By  this  unequal  behaviour  of  bodies  to  this  oxidizing  salt,  it  is  suitable  for 
the  detection  of  many  substances.  Either  changes  of  color  only,  or  also 
the  separation  of  precipitates,  are  thereby  observed,  "which  latter  consist 
frequently  of  a  combination  of  oxide  of  chromium  with  a  newly- formed 
product  of  oxidation.  Frequently,  peculiar  odorous  volatile  oxidation  pro- 
ducts are  formed,  which  are  readily  detected  by  reagents.  When  the  salt 
does  not  act  on  a  solution  of  one  or  more  bodies  even  at  the  boiling  heat 
of  water,  a  little  sulphuric  acid  is  then  added,  by  which  an  oxidation  is 
effected,  except  in  a  few  cases.  Only  a  few  bodies  withstand  the  action  of 
chromic  acid,  which  is  rendered  free  by  the  addition  of  sulphuric  acid. 
The  experiment  with  bichromate  of  potash,  as  well  as  with  chromic  acid, 
is  best  undertaken  in  a  retort,  or  a  flask  connected  with  a  refrigeratory 
apparatus  and  a  receiver,  to  intercept  the  bodies  which  are  volatile.  Sal- 
icylic acid,  kinone,  &c,  can  be  thus  easily  detected  in  the  distillate.  Sub- 
stances which — as,  for  example,  limonine — withstand  the  action  of  chromic 
acid  even  at  high  temperatures,  are  easily  isolated  from  all  other  accom- 
panying constituents,  which  thereby  undergo  decompositions. 
15.  Caustic  alkalies,  the  hydrates  of  potash  and  soda,  as  well  as  baryta, 
are  often  very  serviceable  as  reagents,  partly  in  the  form  of  an  aqueous, 
and  partly  in  the  solid  condition.  Substances  are  dissolved  by  a  solution 
of  an  alkaline  hydrate  which  are  insoluble  in  a  free  state  in  water  or  spirit ; 
while  other  substances,  insoluble,  se,  but  dissolved  in  the  form  of  a 
compound,  are  separated  and  precipitated  by  the  alkaline  solution.  Potash, 
soda,  and  baryta,  effect  changes  by  bodies  being  heated  in  their  solutions, 
which  often  afford  conclusions  on  the  nature  of  the  bodies,  and  quite  as 
often  draw  attention  to  the  existence  of  a  body  which  otherwise  had  re- 
mained unobserved.  Ammonia,  methylamine,  and  similar  bases,  are  often 
thereby  set  free  from  nitrogenous  bodies,  sometimes  affording  a  decompo- 
sition product  which,  with  the  alkali,  forms  an  insoluble  compound  which 
separates.  This  takes  place  more  frequently  with  baryta,  than  with  soda, 
or  potash.  Indeed,  often  no  such  insoluble  compounds  result,  but  a  sol- 
uble salt  of  a  decomposition  product  is  formed,  which  is,  per  se,  insoluble 
in  water,  so  that  only  after  saturation  with  an  acid  a  decomposition  pro- 
duct is  separated.  As  organic  substances  in  contact  with  the  air  more 
readily  take  up  its  oxygen  in  the  presence  of  strong  bases  than  happens 
without  the  agency  of  these  bases,  it  is  necessary  to  exclude  the  air  when 
the  action  of  alkalies  on  organic  substances  is  to  be  observed.  Conse- 
quently, the  bodies  are  heated  with  alkalies  in  vessels  which  are  filled  with 
