890 
EEV.  T.  P.  DALE  AKD  DE.  J.  H.  GLADSTOXE  ON  THE  INPETEN’CE 
density  at  4°  C.  is  not  without  its  influence  on  the  amount  of  sensitiyeness ; the  change 
of  refractive  index  between  10°  and  5°  being  0‘0002,  while  that  between  5°  and  0°  is 
only  O'OOOl.  This  fact,  however,  will  come  out  more  plainly  on  a reference  to  the 
following  Table,  which  represents  the  refractive  indices  of  one  of  the  series  of  observa- 
tions made  with  especial  view  to  this  point.  The  indices  are  calculated  to  five  places 
of  decimals. 
Tempera- 
1 
Refractive  1 Refractive 
Refractive 
ture. 
index  of  A. 
index  of  D.  index  of  H. 
o°c. 
1-33374 
1 
1-32913 
1-34377 
2 
1-32913 
1-34377 
3 
1-32913 
4 
1-32902 
1-33367 
1-34366 
6-5 
1-33356 
1-34366 
9 
1-32882 
1-33342 
1-34337 
11 
1-32879 
1-34331 
Nor  is  this  the  only  indication  of  the  influence  of  change  in  density  exhibited  by  the 
Table  of  indices  of  water.  It  is  there  evident  that  the  sensitiveness  of  water  is  greater 
at  high  than  at  low  temperatures ; so  is  its  change  of  density ; but  the  two  do  not  pro- 
ceed ]}assu.  It  is  well  known  also  that  water  becomes  much  lighter  specifically 
as  it  freezes ; it  has  also  been  observed  that  the  refractive  index  of  ice  is  much  less  than 
that  of  water.  The  following  determinations  will  show  that  the  two  changes  are 
almost  identical  in  amount,  or  rather  that  the  ratio  between  the  density  and  the  mean 
refraction,  (jjjy — I,  is  almost  constant.  The  refractive  index  of  ice  was  determined  for 
D at  I '3089  and 
Substance. 
Mean 
refraction. 
Specific 
gravity. 
Refractive 
power. 
Ice  at  ...  0° 
Water  at  0° 
0-3089 
0-3330 
0- 9184* 
1- 0000 
2973 
3003 
The  similarity  is  here  so  close  as  to  suggest  the  idea  that  the  slight  discrepancy  is 
due  to  errors  of  experiment.  If  it  be  so,  water  has  not  changed  its  refractive  power  by 
passing  from  the  liquid  to  the  solid  state,  and  another  deduction  of  M.  Jamin  may  be 
made  more  general.  He  found  that  the  refraction  of  water  under  diflerent  pressui'es 
varied  in  precise  accordance  with  the  amount  of  condensation,  and  concluded  that  “ it 
may  be  admitted  that  this  law  of  constancy  of  the  refractive  power  is  worthy  of  accept- 
ance when  the  water  does  not  change  its  physical  condition,  and  remaining  at  the  same 
temperature,  is  submitted  to  variable  pressure.”  The  law  may  prove  to  be  worthy  of 
acceptance  even  when  water  does  change  its  physical  condition  from  the  liquid  to  the 
solid. 
The  refractive  indices  of  the  principal  fixed  lines  were  determined  as  follows  at  a 
temperature  of  15°  C. ; — 
* According  to  PLAVrAia  and  Joule,  Mem.  Chem.  Soc.  vol.  ii. 
