Prof.  Thomson  on  the  Abrupt  Change  at  Boiling  or  Condensing.  229 
Now  it  will  be  my  chief  object  in  the  present  paper  to  state  and 
support  a  view  which  has  occurred  to  me,  according  to  which  it  ap- 
pears probable  that,  although  there  be  a  practical  breach  of  continuity 
in  crossing  the  line  of  boiling-points  from  liquid  to  gas  or  from  gas 
to  liquid,  there  may  exist,  in  the  nature  of  things,  a  theoretical  con- 
tinuity across  this  breach  having  some  real  and  true  significance. 
This  theoretical  continuity,  from  the  ordinary  liquid  state  to  the 
ordinary  gaseous  state,  must  be  supposed  to  be  such  as  to  have  its 
various  courses  passing  through  conditions  of  pressure,  temperature, 
and  volume  in  unstable  equilibrium  for  any  fluid  matter  theoretically 
conceived  as  homogeneously  distributed  while  passing  through  the 
intermediate  conditions.  Such  courses  of  transition,  passing  through 
unstable  conditions,  must  be  regarded  as  being  impossible  to  be  brought 
about  throughout  entire  masses  of  fluids  dealt  with  in  any  physical 
operations.  Whether  in  an  extremely  thin  lamina  of  gradual  trans- 
ition from  a  liquid  to  its  own  gas,  in  which  it  is  to  be  noticed  the 
substance  would  not  be  homogeneously  distributed,  conditions  may 
exist  in  a  stable  state  having  some  kind  of  correspondence  with  the 
unstable  conditions  here  theoretically  conceived,  will  be  a  question 
suggested  at  the  close  of  this  paper  in  connexion  with  some  allied 
considerations. 
It  is  first  to  be  observed  that  the  ordinary  liquid  state  does  not 
necessarily  cease  abruptly  at  the  line  of  boiling-points,  as  it  is  well 
known  that  liquids  may,  with  due  precautions,  be  heated  consi- 
derably beyond  the  boiling  temperature  for  the  pressure  to  which 
they  are  exposed.  This  condition  is  commonly  manifested  in  the 
boiling  of  water  in  a  glass  vessel  by  a  lamp  placed  below,  when  the 
temperature  of  the  internal  parts  of  the  water,  or,  in  other  words, 
of  the  parts  not  exposed  to  contact  with  gaseous  matter,  rises  consi- 
derably above  the  boiling-point  for  the  pressure,  and  the  water  boils 
with  bumping  *.  At  this  stage  it  becomes  desirable  to  refer  to  Dr. 
Andrews's  diagram  of  curves  showing  his  principal  results  for  carbonic 
acid,  and  to  consider  carefully  some  of  the  remarkable  features  pre- 
sented by  those  curves.  In  doing  so,  we  have  first,  in  the  case  of  the 
two  curves  for  13°*1  and  21°*5,  which  pass  through  the  boiling  in- 
terruption of  continuity,  to  guard  against  being  led,  by  the  gradu- 
ally bending  transition  from  the  curve  representing  obviously  the 
liquid  state  into  the  line  seen  rapidly  ascending  towards  the  curve 
representing  obviously  the  gaseous  state,  to  suppose  that  this  curved 
transition  is  in  any  way  indicative  of  a  gradual  transition  from  the 
liquid  towards  the  gaseous  state.  Dr.  Andrews  has  clearly  pointed 
out,  in  describing  those  experimental  curves,  that  the  slight  bend 
*  It  has  even  been  found  by  Dufour  (Bibliotheque  Universelle,  Archives,  year 
1861,  vol.  xii.,  "Kecherches  sur  1' ebullition  des  Liquides)  that  globules  of  water 
floating  immersed  in  oil,  so  as  neither  to  be  in  contact  with  any  solid  nor  with 
any  gaseous  body,  may,  under  atmospheric  pressure,  be  raised  to  various  tem- 
peratures far  above  the  ordinary  boiling-point,  and  occasionally  to  so  high  a  tem- 
perature as  178°  C,  without  boiling. 
On  this  subject  reference  may  also  be  made  to  the  important  researches  of 
Donnv,  "Sur  la  cohesion  des  Liquides  et  sur  leur  adherence  aux  Corps 
Bolide's,"  Ann.  de  Chimie,  year  1846,  3rd  ser.  vol.  xvi.  p.  167.— July  28,  1871. 
