894  ON  THE  INFLUENCE  OF  TEMPEEATUEE  ON  THE  EEFEACTION  OF  LIGHT. 
Tempera- 
Refractive 
Refractive 
Refractive 
Sensitiveness 
Length  of 
spectrum, 
Dispersive 
ture. 
index  of  A. 
index  of  D. 
index  of  G. 
for  5°  (A). 
power. 
25°  C. 
1-5700 
1-5880 
1-6414 
0-0022 
0-0025 
0-0024 
0-0029 
0-0026 
0-0027 
0-0714 
0-1214 
30 
1-5678 
1-6389 
0-0711 
35 
1-5653 
1-5832 
1-6361 
0-0708 
0-1214 
40 
1-5629 
1-5796 
1-6328 
0-0699 
0-1206 
45 
1-5600 
1-6296 
0-0696 
50 
1-5574 
1-6266 
0-0692 
60 
1-5520 
1-5690 
1-6200 
0-0680 
0-1195 
Camphor  in  Alcohol. — Alcohol,  saturated  with  camphor  at  about  25°  C.,  was  rendered 
considerably  more  refractive  and  dispersive,  but  its  sensitiveness  remained  almost  exactly 
the  same. 
A comparison  of  the  observations  here  noted  seems  to  warrant  the  following  con- 
clusions : — 
I.  In  every  substance  the  refractive  index  diminishes  as  the  temperatm'e  increases. 
The  comparative  amount  of  sensitiveness  varies  greatly,  from  0*0042  per  5°  C.  in  phos- 
phorus to  as  little  as  0*0002  in  water. 
II.  The  length  of  the  spectrum  varies  as  the  temperature  increases.  In  the  case  of 
highly  dispersive  bodies,  as  bisulphide  of  carbon  and  hydrate  of  phenyl,  it  decreases 
considerably ; in  the  case  of  less  dispersive  bodies,  as  the  alcohols,  it  decreases  to  a less 
extent  ,*  while  with  water  the  decrease  is  scarcely  appreciable. 
III.  In  some  substances  the  dispersive  power  is  diminished,  in  others  it  is  augmented 
by  a rise  of  temperature ; that  is,  in  such  substances  as  bisulphide  of  carbon,  it  is  the 
numerator  of  the  function  ^ ^ that  decreases  fastest,  while  in  such  substances  as  water 
f*D— 1 
it  is  the  denominator. 
IV.  The  sensitiveness  of  a substance  is  independent  of  its  specific  refi'active  or 
dispersive  power.  Thus  water  and  ether  are  very  similar  as  to  the  actual  amount  of 
the  refraction  and  dispersion  exhibited  by  them  at  the  ordinary  temperature,  but  ether 
is  many  times  more  sensitive  to  heat  than  water  is. 
V.  The  amount  of  sensitiveness  is  not  directly  proportional  to  the  change  of  density 
produced^  by  alteration  of  temperature,  yet  there  is  some  relationship  between  the  two 
phenomena.  This  has  been  already  pointed  out  in  reference  to  water ; and  in  general 
those  substances  that  are  most  affected  in  density  by  heat  are  the  most  sensitive. 
VI.  No  sudden  change  of  sensitiveness  occurs  near  the  boiling-point;  at  least,  this  is 
true  in  respect  to  bisulphide  of  carbon,  ether,  and  methylic  alcohol. 
The  Rev.  T.  P.  Dale  originated  this  inquiry.  He  too  is  responsible  for  the  calcula- 
tions, and  Dr.  Gladstone  for  all  the  angular  measurements  on  which  they  are  based,  and 
the  purity  of  the  substances  employed. 
