ON  THE  IGNITING  POINT  OF  VAPORS,  ETC.  255 
the  percentage  of  spirit  in  it ;  this,  however,  I  have  not  gone 
into.  The  proportion  of  volatile  matters  to  be  found  in  different 
crude  commercial  substances  is  exceedingly  variable,  and  there- 
fore no  line  for  guidance  do  I  offer ;  but  in  manufactured  articles 
of  commerce,  where  a  volatile  and  a  less  volatile  mixture  are  to- 
gether, the  manufacturer  and  the  merchant  have  it  in  their 
power  to  exact  a  standard  at  which  the  vapor  will  not  ignite. 
A  very  small  percentage  of  a  volatile  compound  is  sufficient  to 
make  the  whole  bulk  dangerous,  and  in  some  instances  accidents 
from  this  circumstance  are  very  apt  to  arise.  In  the  printed 
table  I  have  light  pitch  oil,  the  vapor  of  which  explodes  at  119° 
F. ;  this  point  of  ignition  is  not  what  is  considered  at  all  dan- 
gerous as  compared  with  bisulphide  of  carbon  or  benzole ;  it  is, 
however,  equally  dangerous,  and  for  this  reason — that  the  latter 
is  known  to  give  off  inflammable  vapor  which  ignites  at  a  low 
temperature,  while  the  former,  on  account  of  its  familiar  name 
— pitch,  oil,  or  creosote — is  looked  upon  as  not  at  all  explosive. 
In  this  sample  of  light  pitch  oil,  the  volatile  matter  which  gave 
off  inflammable  gas  at  119°  did  not  exceed  2  per  cent.,  after 
which  no  combustible  vapor  was  given  off  until  a  temperature  of 
180°  was  reached,  thus  clearly  showing  that  the  low  explosive 
points  of  the  vapors  of  some  commercial  substances  depend  upon 
a  very  small  percentage  of  volatile  extraneous  matter. 
Now  I  shall  explain  the  small  apparatus  used  in  estimating 
the  igniting  point  of  the  vapors,  and  which  is  very  simple. 
It  consists  of  a  water-bath,  with  basin  thermometer  and  spirit 
lamp.  In  operating,  I  put  the  same  quantity  of  cold  water  into 
the  bath  each  trial,  in  order  that  the  time  required  to  raise  the 
temperature  of  the  water  is  as  nearly  as  possible  the  same.  Into 
the  small  basin  I  put  a  known  measure  of  the  liquid  under  ex- 
amination (in  this  instance,  also,  the  same  volume  is  always 
used) ;  the  thermometer  is  then  adjusted  with  the  bulb  immersed 
under  the  liquid  in  the  basin.  The  spirit  lamp  is  now  liglited 
and  placed  under  the  bath — the  water  in  the  bath  is  gradually 
warmed,  which,  in  its  turn,  heats  the  liquid  under  trial.  The 
rise  of  temperature  is  indicated  by  the  thermometer,  and  by 
means  of  a  lighted  taper  and  careful  attention  it  is  easy  to  catch 
the  first  flash  of  vapor  evolved.    In  order  to  have  exact  com- 
