Lignite  is  a  brown  coal  of  more  recent  formation  than  the  two  pre-
ceding varieties.  It  frequently  retains  the  structure  of  the  wood  from
which  it  was  formed  and  contains  from  50  to  80  per  cent,  of  carbon  and
15  to  20  per  cent,  of  moisture.
Jet  is  a  variety  of  this  brown  coal,  so  compact  as  to  take  a  fine  polish.
Silicon  Carbide,  or  Carborundum,  SiC,  is  made  by  heating  a  mix-
ture of  34  per  cent,  coke,  54.2  per  cent,  sand,  9.9  per  cent,  sawdust,  and
1.7  per  cent,  of  common  salt  in  an  electric  furnace  to  about  3500°;
SiOj  +  30  =  SiC  +  2CO.  When  pure,  silicon  carbide  is  colorless,  the
crystals  obtained  in   the  commercial   manufacture  are  blue,  black,  or


CARBON  AND  HYDROGEN.  281

brown,  with  beautiful  iridescence.  It  is  insoluble  in  ordinary  solvents
and  is  only  attacked  by  fused  caustic  alkalies  and  their  carbonates.
In  hardness,  carborundum  ranks  next  to  boron  carbide  and  the
diamond;  hence  its  use  as  an  abrasive,  the  crystals  being  crushed  and
moulded  into  the  desired  forms.
The  Metallic  Carbides. — ^These  are  compounds  of  the  various
metals  and  carbon  obtained  through  solution  of  carbon  in  the  fused
metal,  or  more  usually  by  the  reduction  of  metallic  oxides  with  carbon,
by  means  of  the  high  temperature  of  the  electric  furnace.  According  to
their  d<$ortment.  to  water,  they  may  be  divided  into  the  following
classes:

1.  Those  decomposed  by  cold  water  :  the  carbides  of  lithium  (LijCj),
potassium,  calcium,  barium  (BaC^),  strontium,  aluminum  (Al^Cj),
manganese  (MnC),  uranium  (U2C3),  etc.
2.  Those  not  decomposed  by  water:  chromium  (Cr^C.CrjCj),  molyb-
denum  (MoC),  titanium,  boron,  etc.
Aluminum  carbide  yields  methane;

AUCs       +     i2HaO     =     3CH4     4-     4A1(0H)8.
Uranium  carbide  yields  methane,  ethane  and  hydrogen.
