2.28  Royal  Society : — 
different  in  density  and  other  properties  while  the  temperature  and 
pressure  are  the  same  in  both,  and  when  we  find  also  that  an  intro- 
duction or  abstraction  of  heat  without  change  of  temperature  or  of 
pressure  will  effect  the  change  from  the  one  state  to  the  other,  and 
also  find  that  the  change  either  way  is  perfectly  reversible,  we 
speak  of  the  one  state  as  being  an  ordinary  gaseous,  and  the  other 
as  being  an  ordinary  liquid  state  of  the  same  matter;  and  the  ordi- 
nary transition  from  the  one  to  the  other  we  should  designate  by  the 
terms  boiling  or  condensing,  or  occasionally  by  other  terms  nearly 
equivalent,  such  as  evaporation,  gasification,  liquefaction  from  the 
gaseous  state,  &c.  Cases  of  gasification  from  liquids  or  of  con- 
densation from  gases,  when  any  chemical  alteration  accompanies 
the  abrupt  change  of  density,  are  not  among  the  subjects  proposed 
to  be  brought  under  consideration  in  the  present  paper.  In  such 
cases  I  presume  there  would  be  no  perfect  reversibility  in  the  pro- 
cess ;  and  if  so,  this  would  of  itself  be  a  criterion  sufficing  to  sepa- 
rate them  from  the  proper  cases  of  boiling  or  condensing  at  present 
intended  to  be  considered.  If,  now,  the  fluid  substance  in  the  rarer 
of  the  two  states  (that  is,  in  what  is  commonly  called  the  gaseous 
state)  be  still  further  rarefied,  by  increase  of  temperature  or  dimi- 
nution of  pressure,  or  be  changed  considerably  in  other  ways  by 
alterations  of  temperature  and  pressure  jointly,  without  its  receiving 
any  abrupt  collapse  in  volume,  it  will  still,  in  ordinary  language  and 
ordinary  mode  of  thought,  be  regarded  as  being  in  a  gaseous  state. 
Remarks  of  quite  a  corresponding  kind  may  be  made  in  describing 
various  conditions  of  the  fluid  (as  to  temperature,  pressure,  and 
volume)  which  would  in  ordinary  language  be  regarded  as  belonging 
to  the  liquid  state. 
Dr.  Andrews  (Phil.  Trans.  1869,  p.  575)  has  shown  that  the 
ordinary  gaseous  and  ordinary  liquid  states  are  only  widely  separated 
forms  of  the  same  condition  of  matter,  and  may  be  made  to  pass  into 
one  another  by  a  course  of  continuous  physical  changes  presenting 
nowhere  any  interruption  or  breach  of  continuity.  If  we  denote 
geometrically  all  possible  points  of  pressure  and  temperature  jointly, 
by  points  spread  continuously  in  a  plane  surface,  each  point  in  the 
plane  being  referred  to  two  axes  of  rectangular  coordinates,  so  that 
one  of  its  ordinates  shall  represent  the  temperature  and  the  other 
the  pressure  denoted  by  that  point,  and  if  we  mark  all  the  successive 
boiling-  or  condensing-points  of  temperature  and  pressure  as  a  con- 
tinuous line  on  this  plane,  this  line,  which  may  be  called  the  boiling- 
line,  will  be  a  separating  boundary  between  the  regions  of  the  plane 
corresponding  to  the  ordinary  liquid  state  and  those  corresponding 
to  the  ordinary  gaseous  state.  But,  by  consideration  of  Dr.  Andrews's 
experimental  results,  we  may  see  that  this  separating  boundary  comes 
to  an  end  at  a  point  of  pressure  and  temperature  which,  in  conformity 
with  his  language,  may  be  called  the  critical  point  of  pressure  and 
temperature  jointly;  and  we  may  see  that,  from  any  ordinary  liquid 
state  to  any  ordinary  gaseous  state,  the  transition  may  be  effected 
gradually  by  an  infinite  variety  of  courses  passing  round  outside  the 
extreme  end  of  the  boiling- line. 
