Prof.  Thomson  on  the  Abrupt  Change  at  Boiling  or  Condensing.  233 
admitting  speedily  of  dry  steam  nearly  at  its  condensing  temperature 
for  its  pressure  (or,  to  use  a  common  expression,  nearly  saturated) 
into  a  vessel  with  a  piston  or  plunger,  all  kept  hotter  than  the  steam, 
and  then  allowing  the  steam  to  expand  till  by  its  expansion  it  would 
be  cooled  below  its  condensing-point  for  its  pressure  ;  and  yet  I  should 
suppose  that  if  this  were  done  with  very  careful  precautions  the  steam 
might  not  condense,  on  account  of  the  cooled  steam  being  surrounded 
entirely  with  a  thin  film  of  superheated  steam  close  to  the  super- 
heated containing  vessel.  The  fact  of  its  not  condensing  might 
perhaps  best  be  ascertained  by  observations  on  its  volume  and  pres- 
sure. Such  an  experiment  as  that  sketched  out  here  would  not  be 
easily  made ;  and  unless  it  were  conducted  with  very  great  precau- 
tions, there  could  be  no  reasonable  expectation  of  success  in  its  at- 
tempt ;  and  perhaps  it  might  not  be  possible  so  completely  to  avoid 
the  presence  of  dust  or  other  dense  particles  in  the  steam  as  to  make 
it  prove  successful.  I  mention  it,  however,  as  appearing  to  be  founded 
on  correct  principles,  and  as  tending  to  suggest  desirable  courses  for 
experimental  researches.  The  overhanging  part  of  the  curve  from  e 
to/ seems  to  represent  a  state  in  which  there  would  be  some  kind 
of  unstable  equilibrium ;  and  so,  although  the  curve  there  appears 
to  have  some  important  theoretical  significance,  yet  the  states  repre- 
sented by  its  various  points  would  be*  unattainable  throughout  any 
ordinary  mass  of  the  fluid.  It  seems  to  represent  conditions  of  co- 
existent temperature,  pressure,  and  volume  in  which,  if  all  parts 
of  a  mass  of  fluid  were  placed,  it  would  be  in  equilibrium,  but  out 
of  which  it  would  be  led  to  rush,  partly  into  the  rarer  state  of  gas,  and 
partly  into  the  denser  state  of  liquid,  by  the  slightest  inequality  of 
temperature  or  of  density  in  any  part  relatively  to  other  parts.  I 
might  proceed  to  state,  in  support  of  these  views,  several  considera- 
tions founded  on  the  ordinary  statical  theory  of  capillary  or  super- 
ficial phenomena  of  liquids,  which  is  dependent  on  the  supposition 
of  an  attraction  acting  very  intensely  for  very  small  distances,  and 
causing  intense  pressure  in  liquids  over  and  above  the  pressure  ap- 
plied by  the  containing  vessel  and  measurable  by  any  pressure- 
gauge.  That  statical  theory  has  fitted  remarkably  well  to  many 
observed  phenomena,  and  has  sometimes  even  led  to  the  forecasting 
of  new  results  in  advance  of  experiment.  Hence,  although  dyna- 
mic or  kinetic  theories  of  the  constitution  and  pressure  of  fluids 
now  seem  likely  to  supersede  any  statical  theory,  yet  phenomena 
may  still  be  discussed  according  to  the  principles  of  the  statical  theory  ; 
and  there  may  be  considerable  likelihood  that  conditions  explained 
only,  and  in  continuous  contact  with  one  another,  or  in  contact  only  under  spe- 
cial circumstances  with  other  substances,  experience  a  difficulty  of  making  a 
beginning  of  their  change  of  state,  whether  from  liquid  to  solid,  or  from  liquid 
to  gaseous,  or  probably  also  from  solid  to  liquid,"  was  proposed  by  me,  and,  so 
far  as  I  am  aware,  was  first  announced  in  a  paper  by  me  in  the  Proceedings  of  the 
Koval  Society  for  November  24, 1850  (vol.  x.  p.  158),  and  in  a  paper  submitted 
to  the  British  Association  in  the  same  year. 
In  the  present  paper,  at  the  place  to  which  this  note  is  annexed,  I  adduce 
the  like  further  supposition  that  a  difficulty  of  making  a  beginning  of  change 
of  state  from  gaseous  to  liquid  may  also  probably  exist. 
