446  PREPARATION  OF  HYDROFLUORIC  ACID. 
than  at  12°  C,  and  so  on :  the  difference  is  asserted  to  be  mate- 
rial. Fresenius'  experiments  were  made  with  sulphuric  acid 
and  hypo-nitric  acid,  and  the  delicacy  of  the  reaction  obtained 
by  him  at  corresponding  temperatures  seems  to  fall  a  little  short 
of  the  above. — Am.  Jour.  Science  and  Arts,  July,  1866. 
ON  THE  PREPARATION  OF  HYDROFLUORIC  ACID. 
By  W.  P.  Dexter. 
Few  chemists  have  at  their  disposal  a  distillatory  apparatus  of 
platinum,  and  the  cryolite  from  which  the  purest  hydrofluoric 
acid  is  prepared ;  and  the  shape  commonly  given  to  the  dome  of 
platinum  retorts  is  such  as  to  allow  matter  which  may  be  pro- 
jected upon  it  to  flow  down  the  neck  into  the  receiver.  The  acid 
made  from  fluor  spar  in  such  a  retort  I  have  found  to  contain 
sulphate  of  lime.  A  simple  remedy  for  this  defect  would  be  to 
fix  in  the  dome  a  perforated  disc,  or  ring,  over  the  aperture  of 
which  another  disc  of  less  diameter,  but  larger  than  this  aperture, 
is  supported  by  three  strips  of  platinum  riveted  to  the  ring,  both 
being  made  of  silver  platinum  foil. 
A  dome  of  platinum  attached  to  a  leaden  vessel  seems  to  me  a 
half-way  measure,  combining  the  disadvantages  attending  the  use 
of  both  metals. 
To  those  who  are  not  in  possession  of  an  apparatus  of  platin- 
um, I  can  recommend  from  experience  the  following  compara- 
tively inexpensive  arrangement. 
It  consists  of  the  ordinary  leaden  bore,  (mine  is  6"  high  by 
8J"  internal  diameter,)  made  of  a  piece  of  lead  pipe  into  which 
a  bottom  of  lead  is  cast,  and  provided  near  the  top  with  a  small 
and  short  tube  for  the  escape  of  the  gas.  The  tube  must  incline 
slightly  from  the  retort  upward,  otherwise  whatever  is  condensed 
or  projected  upon  it  will  flow  downward  and  the  product  be  con- 
taminated, at  least,  with  lead.  Into  this  tube  a  smaller  one  of 
platinum  is  luted,  which  is  bent  into  the  shape  of  a  quarter  of  a 
circle  so  that  the  farther  end  points  downwards ;  this  end  is 
soldered  with  gold  into  the  bottom  of  an  inverted  platinum  cru- 
cible. An  old  one,  perforated  and  cracked,  such  as  is  gen- 
erally to  be  found  in  a  laboratory,  answers  perfectly  for  the 
purpose. 
