592 Scientific Intelligence. 
for all the other gases. That they are of the same order of mag- 
nitude, except for binoxide of nitrogen and carbonic oxide, while 
in no cases are they exactly equal; 
Gas. B’x10® | $Bx10°| Bx10* | 10%(n—1)| Cx10* 
Hydrogen ..-..--..| — 57 —87 381 1388 44 
ee es St oy oe +107 | +72 383 2923 58 
Hitrogen 23 cuss 85 382 2972 69 
Protoxide nitrogen.) 754 880 388 5084 | 127 
Binoxide nitrogen _| 225 70 367 2967 
Carbonic oxide_.-. 435 89 367 3336 75 
Carbonic acid - _.-- 901: 720 406 4494 52 
Sulphurous acid _._| 3220 2500 471 6820 
Cyanogen ______-. 3220 2700 8202 100 
This difference may be in part due to the difficulty of their deter- 
mination, to differences of temperature, and to want of exactness 
of the formulas employed. Hence we may say that at a constant 
temperature the excess of refraction n—1 of a gas is nearly pro- 
portional to the density. ; 
e same law does not hold as we change the temperature; if 
J is the number of bands which pass when the pressure varies fro 
one value to another, f (1+ a¢), instead of being constant, contin- 
ually diminishes as the temperature rises, so that the usual coeffi- 
cient a must be replaced by a larger one £, whose value multiplied 
by 100,000 is given for temperatures below 40° in column four of 
the above table. In the case of carbonic acid and binoxide of 
nitrogen # has the same value as a. 
To find the absolute index of refraction it is sufficient to meas- 
ure the length of the tube and to know the value of the wave 
measuring C’ in the formula of Cauchy n-1=A(14-47). These 
values multiplied by 10,000 are given in column six of the table. 
To show more exactly the order of magnitude of the dispersion 
of gases compared with that of other well known bodies, the value 
_of the dispersive power wae has been computed between the 
rays B and H of the solar spectrum. For air the result is 024, 
for the ordinary ray in quartz -032, for water -040, for crown glass 
_ 038, for protoxide of Sapte 053, for heavy flint glass 69. 
Compte Zam Ixxviii, 617, 679. : E, OC. 
0: Fi 
