the  Mechanical  Theory  of  Heat.  107 
Then,  in  1849,  W.  Thomson  published  his  interesting  me- 
moir, "  An  Account  of  Carnot's  Theory  of  the  Motive  Power  of 
Heat,  with  Numerical  Results  deduced  from  Regnault's  Experi- 
ments on  Steam  "*.  In  this  treatise  he  still  adheres  entirely  to 
Carnot's  view,  that  heat  can  perform  work  without  the  quantity 
of  heat  present  being  altered.  It  is  true,  he  adduces  a  difficulty 
which  opposes  this  view,  and  says  (p.  545),  "  It  might  appear 
that  the  difficulty  would  be  entirely  avoided  by  abandoning  Car- 
not's  fundamental  axiom — a  view  which  is  strongly  urged  by 
Mr.  Joule."  But  he  adds,  "  If  we  do  so,  however,  we  meet  with 
innumerable  other  difficulties,  insuperable  without  further  expe- 
rimental investigation  and  an  entire  reconstruction  of  the  theory 
of  heat  from  its  foundation.  It  is  in  reality  to  experiment  that 
we  must  look,  either  for  a  verification  of  Carnot^s  axiom  and  an 
explanation  of  the  difficulty  we  have  been  considering,  or  for  an 
entirely  new  basis  of  the  theory  of  heat." 
At  the  time  of  the  appearance  of  this  memoir,  I  wrote  my  first 
on  the  mechanical  theory  of  heat,  which  was  read  in  the  Berlin 
Academy  in  February  1850,  and  was  printed  in  the  March  and 
April  numbers  of  PoggendorfPs  Annalen,  and  of  which  an  English 
translation  appeared  in  the  Phil.  Mag.  S.  4.  vol.  ii. 
In  it  I  ventured  to  commence  that  reconstruction  of  the  theory 
of  heat,  without  waiting  for  further  experimental  investigation ; 
and  therein,  I  believe,  I  so  far  overcame  the  difficulties  men- 
tioned by  Thomson  that  the  path  was  smoothed  for  all  further 
investigations  of  this  sort. 
In  the  first  part  of  the  memoir,  which  treated  of  the  theorem 
of  the  equivalence  of  heat  and  work,  I  showed,  first,  that  several 
quantities  occurring  in  the  science  of  heat  required  a  conception 
and  treatment  quite  different  from  those  which  they  had  hitherto 
received. 
The  amount  of  heat  which  a  body  must  receive  in  order,  from 
a  given  initial  condition,  to  arrive  at  another,  and  which  was 
named  the  total  heat  of  the  body,  had  previously  been  universally 
treated  as  a  quantity  which  was  perfectly  determined  by  the  mo- 
mentary state  of  the  body;  and  accordingly  it  was  represented 
by  a  function  of  the  volume  and  temperature,  or  of  the  volume 
and  pressure.  I  now  showed  that  such  a  mode  of  representation 
is  inadmissible,  this  quantity  depending  not  merely  on  the  mo- 
mentary state  of  the  body,  but  also  on  the  way  in  which  it  has 
arrived  at  that  state. 
Of  the  so-called  latent  heat,  I  maintained  that  it  no  longer 
exists  as  heat,  but  has  been  expended  for  the  production  of  work. 
I  distinguished  the  work  into  internal  and  external,  and  made 
evident  an  essential  difference  between  the  two — the  former  being 
*  Transactions  of  the  Royal  Society  of  Edinburgh,  vol.  xvi.  p.  541. 
