NOTE  ON  DISTILLED  SULPHURIC  ACID. 
447 
patented  his  invention  in  this  country.  The  nature  of  the  pro- 
cess, which  has  been  for  some  time  an  object  of  much  specula- 
tion, is  extremely  simple,  and  is  mechanical,  not  chemical.  The 
change  which  takes  place  in  pictures,  he  says,  is  "  the  discon- 
tinuance of  molecular  cohesion,"  which  "  process  begins  on  the 
surface  with  microscopical  fissures  in  the  varnish,  and  pene- 
trates by-and-by  through  the  different  coats  of  colors  to  the 
very  foundation.  The  surface  and  body  of  such  a  picture  be- 
come, in  the  course  of  time,  intimately  mixed  with  air,  and 
reflect  light  like  powdered  glass,  or  loses  its  transparency  like 
oil  intimately  mixed  with  water  or  air."  The  process  consists 
in  causing  these  molecules  to  re-unite,  which  he  does  as  fol- 
lows :  The  picture  is  exposed  in  a  flat  case,  lined  with  metal, 
to  an  atmosphere  saturated  with  vapor  of  alcohol  at  the  ordi- 
nary temperature,  which  vapor  is  absorbed  by  the  resinous 
particles  of  the  picture  to  the  point  of  saturation.  The  differ- 
ent separated  molecules  thus  "  re- acquire  cohesion  with  each 
other,  and  the  optical  effect  of  the  original  is  restored  solely  by 
self-action,  the  picture  not  getting  touched  at  all."  Other  sub- 
stances besides  alcohol — such  as  wood-naphtha,  ether,  sulphuric 
and  other  ethers,  turpentine,  petroleum,  benzine,  &c. — may  be 
used.  The  process  seems  to  have  been  very  successful  at  Mu- 
nich, where  Professor  Pettenkofer  restored  some  almost  invisi- 
ble pictures  to  very  nearly  their  original  freshness.  Liebig 
has  reported  favorably  on  the  method,  and  has  given  it  as  his 
opinion  that  it  cannot  injure  the  paintings,  which,  indeed,  is 
almost  a  consequence  of  its  extremely  simple  nature, — Ckem. 
News,  (London),  June  18,  1864,  from  the  Reader. 
NOTE  ON  DISTILLED  S^PHURIC  ACID. 
By  Prof.  xCedwood. 
The  sulphuric  acid  of  the  British  Pharmacopoeia  differs  from 
that  hitherto  used  in  medicine  in  this  country.  We  have  been 
accustomed  under  the  name  of  sulphuric  acid  to  use  commercial 
oil  of  vitriol,  and  this,  according  to  the  last  London  Pharma- 
copoeia, was  to  have  a  specific  gravity  of  P843.  We  are  now 
directed  in  the  British  Pharmacopoeia  to  use,  not  commercial 
