Dr. Apjohn on a 7ie'w Compound. 113 



rays in mica, the result would be the same. The experiments 

 in a subsequent part of this paper may serve to guide us in 

 our choice. Meanwhile, I would observe, that, supposing 

 the above results to be explained on the supposition that o—e 

 is smaller, instead of A greater for heat than for light, it is 

 equivalent to supposing the doubly refracting energy weaker, 

 or a greater thickness of a crystal required to produce a given 

 effect. Our suggestion respecting the existence of sensible 

 vibrations normal to the wave surface (art. 28) will not avail 

 us here. For, by the mode of reducing the experiments on 

 depolarization, the unpolarized part of the heat does not enter 

 into consideration at all * ; consequently those parts of the 

 total effect which are due to transverse vibrations alone, are 

 not modified by double refraction as so much light w^ould be. 



[To be continued.] 



XVI. On a neno Compound, consisting of Iodide of Potassium, 

 Iodine, and the Essential Oil of Cinnamon. By James 

 Apjohn, M.D., M.R.I.A., Professor of Chemistry in the 

 Royal College of Surgeons, Ireland.^ 



T^HE compound which is the subject of the present com- 

 -^ munication owes its origin to an unchemical medical pre- 

 scription. A solution of iodine and iodide of potassium in 

 cinnamon water having been directed by a physician of this 

 city in the winter of 1837, his patient found that during the 

 prevalence of very cold weather, the solution, which had been 

 previously turbid, became quite clear, and nearly insipid, 

 and upon examining the bottle closely he observed deposited 

 in the bottom a small quantity of minute capillary cry- 

 stals. These crystals were brought to Mr. Moore of Anne- 

 street, the apothecary in whose establishment the prescrip- 

 tion was made up, and by him to me for chemical exami- 

 nation and analysis. Before detailing the means which I 

 have employed for determining the exact constitution of this 

 substance it will be proper to give the process by which it 

 is best procured, and enumerate its leading properties ; points, 

 both of which were investigated by Mr. Moore and myself 

 conjointly. 



* I do not mean to offer any opinion on the nature of light in a partially 

 polarized ray generally ; but, as in the present case, the angle of incidence is 

 that of complete polarization nearly, I presume that the transmitted ray 

 is undoubtedly composed partly of light polarized perpendicularly to the 

 plane of incidence, and partly of common light. 



t Communicated by the Author. 



Phil. Mag. S.3. Vol. 13. No. 80. Aug. 1838. I 



