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XXX.    On  a  Non-leaking  Glass  Tap. 
By  A.  P.  Chattock  *. 
IT  is  a  comparatively  simple  matter  to  prevent  the  leakage 
of  gas  between  the  inside  o£  a  glass  tap  and  the  atmo- 
sphere ;  careful  grinding  and  greasing  being  usually  sufficient 
without  the  additional  security  of  mercury  traps  outside  the 
ends  o£  the  plug.  But  to  prevent  the  passage  of  gas  from 
one  side  of  the  plug  to  the  other  is  not  so  easy. 
As  is  well  known,  when  the  plug  revolves  the  edges  of  the 
hole  bored  through  it  tend  to  remove  part  of  the  grease,  and 
thus  to  leave  a  trace  in  the  latter  which  is  only  too  likely  to 
connect  the  opposite  sides  of  the  tap;  and  this  result,  though 
rendered  less  likely,  is  not  by  any  means  certainly  prevented 
by  boring  the  hole  obliquely. 
An  ingenious  tap,  in  which  the  plug  is  hollow  and  contains 
mercury,  was  described  by  Dr.  Milner  in  this  Magazine 
(July  1903,  p.  78).  His  tap  requires  no  grease,  and  abso- 
lutely prevents  the  passage  of  gas  past  the  plug  ;  but  this  is 
in  one  direction  only ;  and  the  tap  is  besides  somewhat 
elaborate.  It  is  the  object  of  this  communication  to  show 
that  by  a  modification  of  Dr.  Milner's  idea  it  is  possible  to 
construct  a  simple  tap  which  shall  prevent  the  passage  of 
gas  in  either  direction,  provided  the  ordinary  use  of  grease  is 
permissible. 
The  figure  shows  the  arrangement  in  section,  the  mercury 
being  marked  black.     It  will  be   seen  that  if  a  leak  occurs 
the  gas  may  get  into  the  plug  ;  but  it  cannot  get  out  again 
when  the  holes  a,  a  are  below  the  mercury. 
*  Communicated  bv  the  Author. 
2  C  2 
