678  Mr.  E.  Buckingham  on 
over  the  one  first  described  and  has  several  inherent 
disadvantages,  it  was  discarded  in  favour  of  what  might  be 
called  the  series  arrangement. 
In  conclusion,  the  author  wishes  to  thank  Dr.  J.  A.  Fleming, 
F.R.S,,  for  his  kindness  in  permiting  him  to  publish  the 
results  of  these  experiments,  which  were  carried  out  under 
his  direction  at  the  Pender  Laboratory,  University  College, 
London. 
LXI.  Elementary  Notes  on  Thermodynamics:  the  Plug 
Experiment.     By  Edgar  Buckingham  *. 
§1.  npHEOEETICALLY  there  are  many  ways  of  estab- 
_L  fishing  the  thermodynamic  scale  of  temperature, 
but  practically,  experiments  on  the  properties  of  gases  offer 
the  only  mode  of  attacking  the  problem  that  has  been  followed 
with  any  success.  For  this  purpose,  what  we  need  to  know 
is  the  manner  in  which  the  internal  energy  of  the  gas  used 
in  our  standard  gas-thermometer  varies  with  the  volume, 
when  the  temperature  is  constant  ;  and  for  the  most  precise 
information  on  this  point,  we  have,  at  present,  to  refer 
to  the  porous-plug  experiment  or  something  equivalent 
to  it. 
The  use  of  reasoning  based  on  the  second  law  of  thermo- 
dynamics is  to  be  met  with  very  frequently ;  and  we  find,  in 
almost  every  case,  that  it  involves  the  assumption  that  the 
international  hydrogen  scale,  to  which  the  observations  are 
referred,  differs  only  by  an  additive  constant  from  Lord 
Kelvin's  thermodynamic  scale  which  appears  in  the  thermo- 
dynamic formulas.  It  might  be  expected,  therefore,  that  a 
clear  treatment  of  the  plug  experiment  would  be  found  in 
every  text-book  of  thermodynamics,  but  that  expectation  is 
not  justified.  Some  of  our  best  standard  works  hardly 
mention  the  plug  experiment,  and  the  general  impression  one 
gets  from  most  of  the  books  I  have  examined,  is  that  the 
authors  consider  the  subject  either  too  difficult  to  be  discussed 
or  so  simple  as  to  deserve  only  casual  notice.  I  think,  how- 
ever, that  a  clear  and  simple  treatment  of  the  theory  of  some 
method  or  methods  by  which  the  relation  of  the  gas  scale  to 
the  thermodynamic  scale  might  be  found,  is  distinctly  worth 
while,  even  if  there  is  nothing  new  about  the  discussion 
except,  possibly,  the  form  in  which  it  is  put. 
§  2.  Beside  the  original  Joule-Thomson  form  of  the  pluo- 
experiment,   in   which  the   flow   of  gas  through  the  porous 
*  Communicated  bv  the  Author. 
