224      STEAM  APPARATUS  FOR  PHARMACEUTICAL  PURPOSES. 
ings  ;  the  lower  opens  into  the  ash  chamber,  below  the  grate  ;  the 
upper,  into  the  space  around  the  boiler,  so  that  if  the  ash  door 
is  accidentally  left  open  the  draft  is  checked  and  no  injury  can 
result.  The  apparatus  is  air  tight,  the  open  valve  upon  the  left 
affording  the  only  ingress  of  air  to  support  combustion.  This 
vault  is  opened  and  closed  by  steam  pressure  in  the  following  man- 
