PREPARATION  OF  ANHYDROUS  SULPHURIC  ACID.  439 
the  free  iodine  are  also  due  to  some  secondary  reaction  produced 
by  the  oxidizing  action  of  the  nitric  acid. 
The  reaction  which  furnishes  hydriodic  ether  is  a  new  exam- 
ple of  slow  decomposition  between  a  salt  and  an  ether  ;  it  is 
represented  by  the  equation 
C4H50,N05+KI= C4H5I + KO,N05. 
Lond.  Chem.  Gaz.,  May  16,  1859. — From  Comjotes  Rendus,  Feb.  14,  1859. 
ON  THE  PREPARATION  OF  ANHYDROUS  SULPHURIC  ACID. 
By  M.  Osann. 
Nordhausen  sulphuric  acid  has  a  spec.  grav.  of  1-856,  and  only 
boils  at  550°  F.  The  consequence  of  these  two  properties  is 
that  in  the  reoccupation  of  the  space  formed  at  the  bottom  of 
the  retort  by  the  vapors  of  the  acid,  a  percussion  takes  place 
which  may  easily  break  the  retort. 
To  prevent  such  an  accident,  the  author  made  use  of  a  well- 
known  means,  consisting  in  the  introduction  of  a  coiled  up  pla- 
tinum wire  into  the  acid.  It  is  necessary  that  one  end  of  the 
wire  should  touch  the  bottom  of  the  vessel,  and  the  other  pro- 
ject above  the  surface  of  the  fluid.  By  this  means  the  distilla- 
tion of  sulphuric  acid  may  be  effected  over  an  Argand  lamp 
without  any  danger. 
In  this  case  the  author  remarked  that  the  distillate  contained 
a  much  larger  quantity  of  anhydrous  sulphuric  acid  than  is  ob- 
tained by  distillation  without  the  platinum  wire. 
The  receiver  was  placed  in  a  water-bath  at  50°  F.  White 
flakes  were  then  seen  in  the  distillate ;  they  gradually  increased, 
and  it  was  observed  that  about  half  the  acid  became  consoli- 
dated into  a  white  mass  of  anhydrous  sulphuric  acid.  If  the 
receiver  be  taken  out  of  the  water-bath  and  exposed  to  the  air, 
a  portion  of  the  distillate  evaporates,  whilst  the  rest  solidifies  to 
anhydrous  acid.  The  author  makes  the  following  remarks  in 
explanation  of  this  fact. 
The  boiling-point  of  a  fluid  depends  partly  upon  its  individual 
nature,  partly  upon  the  pressure  of  the  atmosphere,  and  lastly, 
in  part  upon  the  pressure  exerted  by  superior  strata  of  fluid 
upon  the  lower  ones,  supposing  the  heat  to  be  applied  from  be- 
low.   If  the  uppermost  stratum  of  the  fluid  be  brought  to  the 
