Mr.  de  luc  on 
44a 
water  by  degrees,  I found  that  the  lamella  of  brafs  pre- 
ferred a little  of  its  lengthening.  I did  not  fufpect  the 
rod  of  glafs,  becaufe  the  elafticity  of  this  fubftance  is 
phyfically  perfect,  and  that  this  quality  feems  to  me  to 
be  proper  to  bring  back  bqdies  to  the  fame  ftate,  when- 
ever the  caufes,  of  what  nature  foever  they  be,  which  have 
drawn  them  from  it,  ceafe : and  I had  foon  after  direct  evi- 
dence of  the  irregularity  not  proceeding  from  the  glafs. 
It  was  eafy  for  me  to  find  the  proportion  of  the  lengths 
of  the  brafs  and  the  glafs,  which,  changing  fuddenly  the 
water,  which  had  the  fame  temperature  as  the  air,  for 
warm  water,  produced  no  difference  in  the  heighth  of 
the  point  upon  which  the  Microfcope  pointed:  which 
fhewed  that  the  two  fubftances  had  been  equally  ex- 
panded in  contrary  directions ; but  when  I afterwards 
gradually 
je  trouvois  que  la  lame  de  leton  confervoit  un  peu  de  fon  allongement.  Je  ne 
fufpedtois  pas  la  branche  de  verre;  vu  que  Felafticite  de  cette  matiere  eft  phy- 
ftquement  parfaite^  et  que  cette  qualite  me  paroit  propre  a ramener  les  corps  au 
meme  etat,  quand  les  caufes,  quelles  qu’elles  foyent,  qui  les  en  ont  tires,  vien- 
nent  a cefler : et  j’eus  lieu  enfuite  de  reconnoitre  immediatement  que  l’irregu- 
larite  ne  venoit  pas  du  verre. 
II  me  fut  aife  de  trouver  la  proportion  des  longueurs  du  leton  etdu  verre  par 
laquelle,  changeant  tout  a coup  l’eau  qui  etoit  a la  temperature  de  Fair,  en  de 
Feauchaude,  il  ne  fe  trouvoit  aucune  difference  dans  la  hauteur  du  point  fur 
lequel  le  Microfcope  vifoit:  ce  qui  montroit  que  les  expanftons  des  deux  matieres 
m fens  contraires  ayoient  ete  egales.  Mais  en  diminuant  enfuite  par  degres  la 
% chaleur 
