The  combination  of  any  of  the  above  forms  of  gas  is  called
mixed  gas.  The  different  constituents  are  united  by  a  mechanical
process.  In  or  near  some  of  the  oil  fields,  gas  is  made  out  of
crude  oil.  The  greater  part  of  the  gas  used  for  lighting  pur-
poses in  this  country  is  the  carbureted  water-gas.  If,  however,
the  price  of  oil  continues  at  its  present  high  level,  water-gas
will  not  have  the  advantage  it  possessed  over  coal  gas,  prior  to
the  European  war,  unless  the  enrichers  are  eliminated.  With
the  eventual  elimination  of  the  enrichers  of  water-gas  (petro-
lum,  cannel  coal,  or  naphtha,  which  are  no  longer  needed)  and
the  increased  use  of  mantles,  the  cost  of  water-gas  lighting  will
be  greatly  reduced.1  Considerable  controversy  is  still  waged
over  the  necessity  of  these  enrichers,  which  are  still  required  by
all  franchises  or  city  ordinances.  Once  the  public  fully  appre-
ciates the  distinction  between  the  light  and  heat  values  together
with  the  use  of  incandescent  gas-lighting,  it  will  accept  the
elimination  of  these  old  requirements.  The  Bureau  of  Mines
has  also  found  a  number  of  cheap  grades  of  coal  that  will  pro-
duce gas  but  give  a  low  heat  value;  this  difficulty  no  doubt
will  eventually  be  overcome  by  chemists.
