12 Dr. Inelis’s Extracts from his Prize Essay on Iodine. 
g ‘Y 
/2 
If we examine the composition of the quantity <~;, we 
shall see that the relation between the length and velocity of 
a wave does not depend merely upon the ratio of the intervals 
of the particles to the length of an undulation, but also upon 
the radius of the sphere of their influence. 
On the last principle, I think we can (as M. Fresnel seems 
to have conjectured*) account for the dispersion of light 
without supposing that the waves move with different velocities 
in the free setherial medium; to which supposition there seems 
to be an insuperable objection+. But I reserve this for future 
consideration. In the mean time I hope Professor Powell 
will favour us, through the medium of your Journal, with 
what he has done towards the verification of M. Cauchy’s 
formula, having told us that he is engaged on the subjectt. 
I am, yours, &c. 
Evesham, Aug. 17, 1835. Joun Tovey. 
P.S. In my next communication I believe I shall be able 
to show that if the vibrations of the particles of zther be each 
decomposed into three rectangular directions, two of which 
are perpendicular to the direction in which the light is pro- 
pagated, and the other parallel to it, the vibrations in any one 
of these three directions may be calculated separately; and 
that a satisfactory reason may be assigned why the vibrations 
in the direction of propagation are insensible. (Airy’s Tract, 
art. 101.) 
September 3, 1835. 
IV. Extracts from a Prize Essay on Iodine. By JAMES 
Ineuis, M.D. 
[Continued from p. 444, and concluded. } 
N COURTOIS shortly after his discovery of iodine 
“e formed the black pulverulent iodide of nitrogen by the 
action of iodine upon hydrous ammonia. Gay-Lussac next 
described another compound of dry ammoniacal gas and iodine, 
which he called the ioduret of ammonia, but which might 
rather be called a hydriodate of an iodide of nitrogen. When 
this compound is thrown into water, decomposition takes 
place, hydriodic acid is found in the water, and the black iodide 
* Airy’s Tracts, p. 285, note. t Lbid. 
_{{ Notices of Professor Powell’s verifications of M. Cauchy’s modifica- 
tion of the undulatory theory, so far as they have yet been made public, 
will be found in Lond. and Edinb, Phil. Mag., vol. vi. p. 374; and last vo- 
lume, p. 293.—Eorr. } 
