1^ KEPORT— 1866. 



On a Fhiid 2yossessinr/ Opposite Eofatory Poivers for Rays at opposite ends of 

 the Spectrum. By Professor John H. Jellett. 



The existence of this fluid was discovered in conducting a series of experiments 

 with a new saccharometer which the author had formerly described to the Eoyal 

 Irish Academy *, and which he now exhibited to the Section. In making use 

 of this instrument, it became necessary to compare the rotatory powers of tlie two 

 well-lcnown species of oil of turpentine, namely : — 1. The American oil of turpen- 

 tine, which is obtained from the Pimis australis of North Carolina ; and 2. the 

 French oil of turpentine obtained from the Phuis maritima of Bordeaux. As 

 these fluids, which are opposite in their rotatoiy powers, are chemically identical, 

 and verj' slightly different in their refractive and dispersive powers, it was natural 

 to expect that no difficulty would be found in determining the relative lengths of 

 two columns of these fluids respectively, which should perfectly compensate each 

 other. Two columns of fluid are said to compensate each other when a ray of 

 polarized light, transmitted successively through these columns, emerges from the 

 second column in the same state in which it entered the first. The actual result, 

 however, was wholly different from this anticipation. When the relative lengths 

 were so determined that the intensity of the light transmitted respectively by the 

 two parts of the analyzer f was the same, the colours of these two spectra were 

 wholly different. In reasoning on the difference of colour, the author was enabled 

 to perceive that the American oil of turpentine was much more highly dispersive 

 of the planes of polarization of the elementary rays than the French oil. It is 

 plain, therefore, that if the lengths of the columns be so proportioned that the 

 rotation maybe the same for tlie mean ray, the more dispersive (in the sense just 

 defined) fluid will turn the plane of polarization of the red ray through a less 

 angle, and that of the violet ray through a greater angle than the less dispersive 

 fluid. Hence, remembering that French oil of tui-pentine is left-handed, and 

 American oil of tui-pentine right-handed, it is plain that if a red ray be trans- 

 mitted through two columns, whose lengths are so proportioned, the total effect 

 will be left-handed rotation ; whereas, if a violet or a blue ray be so transmitted, 

 the effect will be right-handed rotation. As these fluids, being identical in com- 

 position, could scarcely act chemically on each other, the same effects might be 

 expected from a single fluid produced by mixing these two columns. 



This the author found to be, in fact, the case. The rotating fluid was fomied 

 by mixing the two oils in the following proportion J : — 



American oil of turpentine G7 



French oil of tui-pentine 3.3 



"When a column of this fluid, whose length was 4 inches, was traversed by a 

 solar ray which had been previously transmitted through plates of red and blue 

 glass, the rotation produced in the plane of polarization of this, which is the ex- 

 treme red ray, was found to be — 1° 35'. 



Again, when the same column was traversed by a ray which had been pre- 

 viously transmitted through a solution of animoniacal sulphate of copper, the 

 rotation was found to be +2°. 



This phenomenon is best shown with solar light, but it may be shown, though 

 •with less distinctness, with the electric or oxy-calcium lights. 



On Comets, and especially on the Comet of 1811. By Coknelius Vaelet. 



Heat. 



DetermiMition of the Mechaniccd Equivcdent of the Tliermal Unit hy Eivperi- 

 ments on the Heat evolved hy Electric Currents §. By J. P. Joxtle, F.R.S. 



* Proceedings, vol. vii. p. 279. t Ibid. p. .349. 



I The proportion of oils, given above, nuist be luiderstood to refer only to the parti- 

 cular specimens of the oils which were used in making these experiments. The rotatory 

 power of commercial oil of turpentine, more especially that of the American oil, is very 

 variable. § Printed in extenso in the Kcports. 



