374  rochleder's  proximate  analysis. 
water.  Sometimes  the  solutions  even  of  colorless  bodies  are.  strongly 
colored.  Generally,  the  color  of  the  solution  appears  red  ;  more  seldom, 
violet  or  blue.  The  yellow  or  brownish-yellow  color  is  quite  as  frequent 
as  the  red.  By  the  addition  of  water  a  more  or  less  complete  decoloriza- 
tion  then  geneially  ensues,  with  or  without  the  formation  of  a  precipitate. 
Sometimes,  by  the  solution  of  a  substance,  a  so-called  copulated  sulphuric 
acid  is  produced,  which  is  found  when  the  solution,  diluted  with  water,  is 
mixed  with  carbonate  of  baryta,  lime,  or  lead,  and  the  sulphuric  acid  salt, 
insoluble  in  water,  separated  by  filtration  from  the  fluid,  which  is  then 
evaporated,  whereby  the  salt  of  the  copulated  acid  is  left  behind. 
19.  Hydrochloric  acid. — The  concentrated  acid  precipitates  many  organic 
substances  from  their  aqueous  solutions,  but  the  precipitation  is  not  gene- 
rally so  complete  as  that  with  sulphuric  acid.  More  of  the  substance  to  be 
precipitated  remains  dissolved  in  the  hydrochloric  acid  solution  than  is  the 
case  with  the  use  of  sulphuric  acid.  Acetic  acid  prevents  generally  the 
precipitation  more  or  less,  and  sometimes  entirely.  The  tannic  acid  of 
galls,  and  other  analogous  bodies,  may  be  precipitated  by  hydrochloric 
acid.  The  dilute  hydrochloric  acid,  at  the  boiling  heat  of  water,  splits  up 
bodies,  and  in  this  respect  it  is  not  surpassed  by  any  other  reagent.  De- 
compositions of  this  kind  are  most  suitably  undertaken  in  an  atmosphere 
of  carbonic  acid.  A  more  thorough  decomposition,  as  happens  so  readily 
with  sulphuric  acid  at  the  margin  of  the  liquid,  is  not  to  be  feared  with 
hydrochloric  acid.  Hydrochloric  acid  affords  with  these  decomposition 
products  very  little  occasion  for  the  formation  of  bye-products.  Through 
the  separation  of  a  difficultly  soluble  or  insoluble  decomposition  product, 
and  by  the  peculiar  color  of  many  such  products,  the  means  are  given  fre- 
quently to  recognize  and  to  direct  attention  to  a  body.  Concentrated 
hydrochloric  acid,  as  well  as  hydrochloric  acid  gas,  like  concentrated  sul- 
phuric acid,  produce  often  a  conspicuous  coloration  of  bodies,  which  some- 
times disappears  on  the  addition  of  water,  but  sometimes  is  only  slightly 
changed  thereby,  and  in  a  few  cases  undergoes  no  alteration  at  all.  Hydro- 
chloric acid  gas  may  be  employed  for  the  determination  of  the  atomic 
weights  of  substances  with  which  it  combines  ;  as,  for  example,  organic 
bases  and  many  hydrocarbons,  of  which  some  give  therewith  a  crystalline, 
and  others  a  volatile  compound.  In  the  latter  case,  hydrochloric  acid  may 
be  employed  for  the  separation  of  several  such  indifferent  bodies.  Hydro- 
chloric acid  is  consequently  the  most  generally  applicable  means  for  the 
recognition  and  separation  of  organic  materials.^ 
Before  I  pass  to  another  reagent,  I  will  only  just  remark,  that  different 
organic  substances  which  are  split  up  by  hydrochloric  acid  require  an  un- 
equally long  time  for  their  decomposition,  so  that,  by  a  gentle  heat  and  a 
sufficiently  diluted  acid,  one  of  the  compounds  is  often  entirely  decom- 
posed before  the  decomposition  of  the  second  has  begun.  In  this  way,  con- 
sequently, it  is  frequently  possible  to  recognize  two  substances  mixed  with 
