CHEMICAL  AND  PHARMACEUTICAL  MANIPULATIONS.  %\ 
The  instrument  itself,  shown  in  the  following  drawing,  consists  of  an  airT 
thermometer,  b  a,  of  glass,  and  containing  mercury  in  the  lower  part  of  the 
bulb  a,  and  a  portion  of  the  stem  b.  A  tube  of  smaller  diameter,  as  seen 
in  the  figure,  passes  down  the  axis  of  the  tube  b,  the  annular  space  being 
Fig.  387. 
made  air-tight  by  a  small  brass  stuffing-box  b,  which  enables  it  to  be  re- 
tained at  any  required  elevation.  An  air-tight  connection  is  made  at  c  with 
a  piece  of  flexible  caoutchouc  tube,  communicating  with  the  service-pipe  by 
means  of  a  gallows-screw.  The  gas  entering  through  this  channel  passes 
into  the  long  stem  of  the  thermometer,  and  thence  to  the  burner  d. 
In  using  the  instrument,  the  bulb  a  must  be  immersed  in  the  water-bath 
with  the  substance  under  examination,  if  that  means  of  heating  is  employed  ; 
and,  in  the  case  of  an  air-bath  or  hot  press,  it  must  be  placed  in  immediate 
vicinity  of  the  substance,  so  as  to  produce  an  equilibrium  of  temperature 
between  the  air  in  the  bulb  and  the  surrounding  atmosphere. 
The  inventor  thus  explains  its  mode  of  operation.  Supposing,  for  ex- 
ample, that  it  is  required  to  keep  an  object  at  a  temperature  of  100°  F., 
then  the  bulb  of  the  instrument  being  placed  contiguous  to  the  object,  a 
free  supply  of  gas  is  allowed  to  flow  through  the  burner,  and  a  flame  ignited. 
The  heat  soon  begins  to  act  upon  the  air  in  the  bulb,  causing  it  to  expand 
