474  Afr.  djE  Luc  on 
Thermometer,  and  that,  after  having  heated  it  to  70°,  I 
brought  it  back  gradually  to  io°,  the  point  either  re- 
turned exactly  to  the  wire,  or  fo  near  to  it  that  I could  draw 
no  conclulion,from  the  fmall  difference, againft  the  regu- 
larity of  the  return  of  the  glafs  to  its  fame  length  in  the 
fame  temperature.  In  one  of  the- four  experiments  it' 
returned  exactly ; in  a fecond,  I wanted  light  indeed  to 
obferve  this  laft  point,  but  I could  judge  from  the  pre- 
ceding fteps  that  it  would  be  exatft;  in  the  others,  the 
differences  would  have  feemed  to  indicate,  that  the 
glafs  had  retained  fome  part  of  its  lengthening;  but  the 
quantity  was  fo  fmall,  that  even  when  it  was  real,  it 
might  be  confidered  as  null  in  practice. 
Glafs  confequently  is  the  fitted:  fubftance  to  be  made 
ufe  of  as  the  ftandard  of  comparifon  in  experiments 
upon  the  comparative  dilatabilities  of  bodies;  fince  what- 
ever 
perature  de  io°  de  mon  Thermometre, . et  qu’apres Tavoir  echauffee  a 70°je  la 
ramenois  pen  a pen  a xo°,  la  pointe  revenoit  exacfement  au  fil,  ou  fi  pres,  que 
je  n’ai  pu  en  tirer  aucune  eonfequence  centre  la  regularite  du  retour  du,  verre  a 
iameme  longueur  dans  la  me  me.  temperature.  Dans  une  des  quatre  experiences 
ilyrevint  exa£tement;dans  un  autre  le  jour  me  manqua  pourobferyer  ce  dernier 
point,  mais  je  pus  jnger  par  les  pas  precedens,  qu’il  feroit  jufte;  et  dans  les  deux 
autres  les  differences  auroient  indique  que  le  verre  aufli  avoit  conferve  un 
peu  de  fon  allongement : mais  la  quantite  etoit  fi  petite,  que  lors  meme  qu’elle 
feroit  reelle,  on  pourroit  la  regarder  comme  nulle  dans  la  pratique. 
Le  verre  eft  done  la  matiere  la  plus  propre  a fervir  tie  terme  de  comparailon 
dans  les  experiences  fur  les  dilatabilites  comparatives  des  corps;  puifque  les  irregu- 
larity 
