*  rochleder's  proximate  analysis. 
371 
11.  Carbonic  acid,  metallic  oxides  insoluble  in  water,  carbonate  of  lead, 
carbonate  of  silver,  carbonate  of  copper,  and  basic  carbonate  of  lead,  are 
often  employed  for  the  separation  of  bodies.  Carbonate  of  zinc  and  car- 
bonate of  cadmium  are  seldom  serviceable.  Many  substances  expel  the 
carbonic  acid  from  the  carbonates,  and  form  compounds  partly  soluble 
and  partly  insoluble  ;  other  substances  drive  off  the  carbonic  acid  quite  as 
little  at  the  boiling  heat  of  water  as  at  ordinary  temperatures,  whilst  some 
other  bodies  are  able  to  expel  the  carbonic  acid  in  the  heat,  but  certainly 
not  in  the  cold.  When,  therefore,  one  or  the  other  of  the  metallic  car- 
bonates named  are  brought  into  a  solution  of  different  bodies,  compounds 
result  partly  soluble  and  partly  insoluble,  while  a  part  of  the  organic  sub- 
stance remains  dissolved  in  a  free  state.  Many  of  the  dissolved  compounds 
are  precipitated  by  alcohol  from  their  watery  solutions,  and  others  are 
not,  whereby  a  further  separation  is  rendered  possible. 
12.  Bisulphites  of  potash,  soda,  and  ammonia. — These  salts  are  prepared 
in  solution,  as  concentrated  as  possible,  by  conducting  sulphurous  acid 
into  concentrated  solutions  of  the  corresponding  carbonates.  They  are 
partly  employed  to  produce,  with  certain  substances,  compounds  which  are 
easily  separated  from  the  other  constituents,  which  are  not  capable  of  com- 
bining with  alkaline  bisulphites.  This  is  the  case  in  the  separation  of 
the  aldehydes  from  indifferent  bodies  which  possess  equal  solubility  in  va- 
rious fluids  with  the  aldehydes.  They  are  partly  used  with  the  co-opera- 
tion of  heat  to  change  individual  substances,  whilst  others  remain  thereby 
unaltered.  Many  substances  which  possess  the  same  solubility  as  others, 
by  this  change  lose  their  solubility  in  a  fluid,  so  that  the  bodies  remaining 
unchanged  may  now  be  readily  separated  from  the  unaltered  ones. 
It  is  necessary,  when  changes  are  produced  by  the  action  of  the  ma- 
terials which  afford  a  solid  body  by  separation  from  a  fluid,  to  always  ob- 
serve whether  the  separated  substance  contains  or  not  sulphurous  acid  and 
an  alkali.  By  this  it  may  be  concluded  whether  an  insoluble  compound, 
with  alkaline  bisulphites,  results  or  not.  As  Knop  has  shown,  many 
bodies  break  up  by  the  action  of  alkaline  bisulphites  in  the  heat  in  a  man- 
ner analogous  to  the  action  of  free  alkalies  and  acids  in  the  heat.  By  this 
capacity  to  produce  decomposition  products,  the  alkaline  bisulphites  are 
quite  as  useful  in  investigations  as  by  their  capacity  to  form  with  many 
bodies  alkaline  salts  under  the  expulsion  of  sulphurous  acid,  whose  oxida- 
tion by  the  oxygen  of  the  air  is  prevented  by  the  presence  of  sulphurous 
acid,  as  Knop  has  observed  with  gallic  acid  and  analogous  bodies. 
13.  Hypochlorite  of  lime. — The  chlorinated  lime  is  employed  as  watery 
solution  which  has  been  separated  from  the  undissolved  hydrate  of  lime 
by  filtration  through  clean  asbestos.  With  many  organic  substances  it 
produces  characteristic  colorations,  which  are  often  so  very  intense  that  a 
small  quantity  of  a  body  can  be  recognized  thereby  in  the  presence  of  other 
substances.   Lichens  contain  peculiar  crystalline  materials,  some  of  which 
