544  Mr.  de  luc  on 
form  conftrudtion  of  the  meafure,  which  cannot  be  more 
accurately  obtained  than  by  a great  diftance  of  the  fixed 
points ; and  attempt,  however,  by  every  means  poffible, 
to  find  the  proportions  of  this  moft  fimple  and  moft  re- 
gular effedt,  with  its  caufe.  It  is  on  this  account,  that, 
in  my  treatife  on  the  Thermometer,  I have  expofed  all 
the  reafons  which  lead  me  to  believe  that  quickfilver  is 
the  body  whofe  changes  of  bulk  are  moft  proportionate 
to  the  variations  of  heat  which  produce  them,  in  order  to 
affure  to  this  liquid  the  preference  as  a common  meafure 
of  heat : and  that  afterwards,  as  I have  faid  above,  I looked 
for  the  proportions  of  its  progreflion  with  thofe  of  heat 
itfelf. 
But  as  to  the  co-eftedts  which  will  be  indicated  by 
thefe  meafures  of  general  caufes,  unlefs  they  can  be  de- 
termined degree  after  degree  by  experiment,  and  the 
objedts 
ce  qu’on  obtient  plus  furement  par  une  grande  diftance  des  points  fixes;  et 
chercher  cependant  par  tous  les  moyens  poflibles  les  rapports  de  cet  efFet  le  plus 
regulier,  avec  fa  caufe.  C’eft  par  ces  raifons  que  dans  mon  traite  fur  Ie  Ther- 
mometre, j’ai  raftemble  tons  les  motifs  qui  me  portent  a croire  que  le  mercurc 
eft  celui  des  corps  dont  les  changemens  de  volume  font  les  plus  proportionnels 
aux  variations  de  la  chaleur  qui  les  prod uifent;  afin  d’aflurer  a ce  iiquide  la  pre- 
ference pour  la  mefure  commune  de  la  chaleur  : et  qu’enfuite,  comme  je  l’ai  dit 
ci-deffus,  j’ai  cherche  les  rapports  de  fa  marche  avec  celle  de  la  chaleur  elle- 
meme. 
Mais  quant  aux  co- effects  qui  feront  indiques  par  ces  mefures  des  caufes  gene- 
rales,  a moins  qu’on  ne  puifte  les  determiner  degre  par  degre  a Taide  de  l’expe- 
rience, 
