ON  THE  MANUFACTURE  OF  OLEUM  iETHEREUM.  129 
could  be  arrived  at  from  merely  watching  the  temperature.  In 
the  cases  of  ISTos.  5  and  6,  the  temperatures  had  been  brought  to 
310°  and  above  at  the  first  heating.  It  was  therefore  thought 
necessary  to  prevent  the  heat  from  rising  above  305°  at  the 
first  heating.  But,  from  results  since  obtained,  it  was  observed 
that  the  temperature  could  frequently  be  raised  to  315°  and 
over,  without  frothing.  For  this  reason  the  frothing  was 
chiefly  attributed  to  other  causes,  in  conjunction  with  the  tem- 
perature, and  it  was  concluded,  that  where  temperature  alone 
could  be  held  accountable  for  the  frothing,  it  was  generally 
above  320°.  The  presence  of  sulphate  of  lead  may  be  mentioned 
as  one  of  the  principal  cases  of  frothing,  and  this  has  been 
already  observed  and  fully  appreciated  by  Dr.  Squibb,  (see 
Journ.  of  Pharm.,  vol.  v.  3d  series,  p.  197).  Other  circum- 
stances, however,  appear  to  effect  the  same  results.  During 
the  latter  part  of  the  distillation,  copious  quantities  of  a  black 
matter  (thiomelanic  acid)  are  formed,  which  when  the  distilla- 
tion has  proceeded  properly,  forms  a  thick  crust  on  the  top  of 
the  liquid  in  the  retort.  A  portion  of  this  will  frequently  coat 
the  walls  of  the  retort,  and  is,  in  most  instances,  readily  re- 
moved with  a  sponge  and  water  or  alcohol.  Sometimes,  how- 
ever, it  will  adhere  pertinaciously  to  the  bottom,  and  can  only 
be  removed  with  great  difficulty.  When  this  is  allowed  to* 
remain,  it  will  almost  invariably  cause  frothing,  and,  in  many 
cases,  boiling  over.  This  can  be  avoided  by  cleaning  the 
retort  with  sand  and  a  little  ether.  It  is  generally  recommended 
to  allow  the  mixture  of  alcohol  and  sulphuric  acid  to  stand 
twenty-four  hours  before  using  it,  for  the  purpose  of  getting 
rid  of  the  sulphate  of  lead.  In  the  writer's  experience,  this 
time  is  not  long  enough  when  large  quantities  are  used,  from 
two  to  three  days  being  necessary. 
The  following  table  has  been  collected  from  observations 
made  since  the  first  series  of  distillations,  and  may  merit  some 
attention : 
9 
