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ON  PERCHLORIC  ACID  AND  ITS  HYDRATES. 
ON  PERCHLORIC  ACID  AND  ITS  HYDRATES. 
By  Professor  Roscoe. 
(Read  before  the  British  Association.) 
All  the  knowledge  we  possess  of  the  quantitative  relations  of 
perchloric  acid  is  the  determination  of  the  composition  of  the 
potassium  salt,  first  analyzed  by  Stadion  in  1816,  and  afterwards 
by  many  other  chemists.  The  perchloric  anhydrous  acid  has 
not  been  isolated,  and  no  analysis  of  the  aqueous  acid  has  ever 
been  made.  We  can  only  account  for  the  neglect  with  which 
chemists  have  treated  the  highest  and  yet  the  most  stable  of 
the  oxides  of  chlorine,  by  the  fact  that  the  preparation  of  the 
acid  in  large  quantities  has  been  attended  with  great  difficulties. 
The  best  method  for  preparing  aqueous  perchloric  acid  is  to  de- 
compose chlorate  of  potassium  with  hydrofluosilicic  acid,  to  boil 
the  chloric  acid  thus  obtained  down,  which  splits  up  into  lower 
oxides  of  chlorine,  which  escape  in  the  gaseous  state,  impure 
perchloric  acid  being  left  behind,  which  is  purified  by  distilla- 
tion. The  acid  thus  obtained  is  in  appearance  not  to  be  dis- 
tinguished from  oil  of  vitriol,  being  a  colorless,  heavy,  thick, 
oily,  corrosive  liquid,  giving  off  by  heating  dense  white  fumes. 
By  heating  the  aqueous  perchloric  acid  with  four  times  its  volume 
of  concentrated  sulphuric  acid,  the  latter  takes  water  from  the 
first,  dense  white  fumes  are  evolved,  a  yellow  mobile  liquid  distils 
over,  afterwards  thick  oily  drops  appear,  which,  when  coming  in 
contact  with  the  yellow  liquid,  form  white  crystals,  pre- 
viously obtained  by  Serullas,  but  in  such  small  quantities  that 
he  was  unable  to  analyze  that  substance,  which,  prepared  in  this 
way,  contain  always  sulphuric  acid,  and  is  therefore  not  fit 
for  analysis,  and  requires  re-distillation.  Heated,  however,  to 
110°  C,  the  crystals  decompose,  and  split  up  again  in  the 
yellow  liquid,  which  distils  over  at  a  low  temperature,  and  the 
thick  oily  liquid  remains  in  the  retort.  The  yellow  liquid  thus 
obtained  is  pure  perchloric  acid,  a  body  not  known  before,  and 
can  be  obtained  also  by  distilling  one  atom  of  perchlorate  of  po- 
tassium with  four  atoms  of  sulphuric  acid.  In  the  pure  state  it  is 
perfectly  colorless,  but  commonly  slightly  yellow,  owing  to  the 
presence  of  lower  oxides  of  chlorine.  Perchloric  acid  is  one  of 
the  most  powerful  oxidizing  agents  known ;  a  single  drop 
