Prof.  A.  de  la  Rive  on  a  New  Hygrometer.  515 
proposed  by  me  in  1825.  My  paper  on  the  subject  first  ap- 
peared in  the  BibliuthequeUniverselle  of  Geneva  (vol.xxviii.p.285); 
and  the  following  extract  of  it  was  published  the  same  year  by 
Gay-Lussac  in  the  Annates  de  Chimie  et  de  Physique  (vol.  xxx. 
p.  87). 
I  remain,  Gentlemen, 
Yours  &c, 
A.  de  la  Rive. 
Translated  from  the  Annates  de  Chimie  et  de  Physique  (vol.  xxx. 
p.  87). 
"  1  plunge  the  bulb  of  a  delicate  thermometer  into  concen- 
trated sulphuric  acid,  and  then  suddenly  withdraw  it  with  a 
slight  shake  so  as  to  leave  only  a  thin  film  of  acid  on  the  surface 
of  the  bulb.  The  thermometer  immediately  rises  several  degrees ; 
and  after  remaining  an  instant  stationary,  it  begins  to  fall.  I 
next  determine  the  number  of  degrees  the  thermometer  rises, 
under  the  same  circumstances,  when  the  atmosphere  is  com- 
pletely saturated  with  moisture ;  the  difference  between  the  two 
results  will  furnish  the  exact  relation  between  the  tension  of  the 
vapour  contained  in  the  atmosphere  at  the  moment  of  my  obser- 
vation and  its  tension  when  the  atmosphere  is  in  a  state  of  per- 
fect saturation.  For  instance,  the  thermometer,  at  the  moment 
of  the  bulb  being  introduced  into  the  sulphuric  acid,  indicated 
12°  Reaumur.  On  being  withdrawn  and  exposed  to  the  air  it 
rose  to  25°' 5,  or  13^°.  Placed  under  a  receiver  in  an  atmo- 
sphere of  perfect  saturation  at  the  same  temperature  of  12°  it 
rose  to  27°,  or  15°.  Hence  the  ratio  of  13^  to  15  expresses 
the  relation  that  exists  between  the  tension  of  the  vapour  con- 
tained in  the  air  and  the  tension  of  saturation  for  a  tempera- 
ture of  12°.  Now  the  ratio  of  13^  to  15  is  equal  to  that  of 
90  to  100;  and  it  will  be  found,  by  referring  to  Gay-Lussac's 
Table,  published  in  the  Traite  de  Physique  Experiment  ale  et 
Mathematique  of  Biot,  that  the  degree  of  De  Saussure's  hygro- 
meter which  corresponds  to  the  tension  of  90  is  95*43.  Du- 
ring the  above  experiment  the  reading  of  the  same  hygrometer 
was  9550. 
a  For  any  other  temperature  than  that  of  12°  R.,  and  for  the 
same  degree  of  De  Saussure's  hygrometer,  the  rise  of  the  ther- 
mometer will  be  proportional  to  the  temperature  of  the  atmo- 
sphere, since  the  quantity  of  aqueous  vapour  the  air  is  capable 
of  containing  depends  mainly  on  its  degree  of  temperature.  It 
would  therefore  appear  to  be  necessary  to  ascertain  the  variation 
of  temperature,  produced  by  air  in  a  state  of  saturation,  for  each 
degree  of  the  thermometer,  were  it  not  that  a  series  of  experi- 
2  L2 
