196 
Formaldehyde. 
1m.  Jour.  Pliarri'. 
April.  1M8. 
formed  by  the  reunion  of  three  molecules  of  formic  aldehyde,  is  the 
base  of  the  solid  product  used  in  the  form  of  pastilles,  or  in 
powdered  form  as  a  disinfectant. 
Preparation  of  Formaldehyde. — The  process  of  the  discoverer, 
Hoffman,  consisted  of  passing  the  vapor  of  methyl  alcohol,  mixed 
with  a  certain  proportion  of  air,  through  a  tube  of  platinum  heated 
to  dull  redness.  The  resulting  gases  were  condensed  in  a  flask,  and 
in  addition  to  the  methyl  alcohol,  small  quantities  of  formic  alde- 
hyde were  obtained. 
Various  modifications  of  Hoffman's  process  were  brought  out  by 
other  investigators,  but  all  experienced  great  difficulty  in  obtaining 
the  final  product  in  any  considerable  quantity,  and  all  found  that 
the  results  might  be  rendered  null  by  the  most  minute  variation  of 
any  one  of  the  factors  employed. 
The  principal  difficulties  encountered,  were,  first,  the  great 
danger  of  explosion  caused  by  bringing  the  vapors  of  methyl  alco- 
hol in  contact  with  incandescent  metal.  These  explosions  some- 
times occurred  in  the  oxidizing  tube  and  sometimes  in  the  tank 
containing  the  alcohol.  A  second  obstacle  was  the  difficulty  of 
oxidizing  a  large  amount  of  alcohol  at  one  time. 
The  whole  process  of  oxidation  must  be  very  carefully  conducted 
and  the  proportion  of  air  and  methyl  vapor  admitted  to  contact 
with  the  incandescent  material  must  be  very  carefully  adjusted,  as, 
if  oxygen  is  not  present  in  sufficient  quantity,  no  change  takes 
place,  and  the  product  obtained  is  methyl  alcohol,  and,  if  oxygen  is 
in  excess,  the  process  is  carried  too  far,  and  formic  acid  or  carbonic 
acid  is  produced. 
In  an  apparatus  which  I  have  designed  for  producing  formalde- 
hyde in  commercial  quantities,  the  methyl  alcohol  to  be  converted 
is  contained  in  a  circular  steel  tank,  capable  of  resisting  heavy 
pressures ;  in  the  bottom  of  this  tank  is  placed  a  coil  of  copper 
pipe  into  which  steam  at  a  high  temperature  is  admitted  ;  by  this 
means  the  alcohol  in  the  tank  is  boiled,  and  the  resulting  vapor  is 
confined  until  a  pressure  of  75  or  80  pounds  is  reached,  when  it  is 
allowed  to  escape  into  an  air-mixer.  This  mixer  is  so  constructed 
that  an  exactly  determined  quantity  of  air  can  be  intimately  com- 
bined with  the  methyl  vapors.  From  the  mixer  the  combination 
of  air  and  vapor  passes  with  great  force  and  velocity  through  a  fine 
tube  into  the  converter,  which  has  been  brought  to  a  dull  red  heat. 
