150 



to three of water, as the solvent medium, aud to employ the smallest 

 possible quantity of the tincture of iodine as the reagent, and after 

 applying heat for a short time, to set in repose. On spontaneous 

 evaporation or cooling, the optical crystals deposit themselves, and 

 may be recognized by the polarizing microscope, according to the 

 description given of this substance in a former notice to the Society 

 in June last. 



You may remember that this proposition was also contained in 

 my paper on iodo-strychnia, which was withdrawn from the Royal 

 Society by me in June last in consequence of a necessity for revision 

 and the completion of experiments requisite to settle the formula of 

 that peculiar substance, and the introduction of an abstract of the 

 literature concerning it. 



I remain, &c., 



W. BIRD HERAPATH. 



IV. " Dynamical Illustrations of the Magnetic and the Heli9oi- 

 dal Rotatory Effects of Transparent Bodies on Polarized 

 Light." By Professor W. THOMSON, F.R.S. Received 

 May 10, 1856. 



The elastic reaction of a homogeneously strained solid has a 

 character essentially devoid of all heli9oidal and of all dipolar asym- 

 metry. Hence the rotation of the plane of polarization of light 

 passing through bodies which either intrinsically possess the heli9oi- 

 dal property (syrup, oil of turpentine, quartz crystals, &c.), or have the 

 magnetic property induced in them, must be due to elastic reactions 

 dependent on the heterogeneousness of the strain through the space 

 of a wave, or to some heterogeneousness of the luminous motions* 

 dependent on a heterogeneousness of parts of the matter of lineal 

 dimensions not infinitely small in comparison with the wave length. 

 An infinitely homogeneous solid could not possess either of those 



* As would be were there different sets of vibrating particles, or were Rankine's 

 important hypothesis true, that the vibrations of luminiferous particles are directly 

 affected by pressure of a surrounding medium in virtue of its inertia. 



