THE 



LONDON AND EDINBURGH 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



JULY 1836. 



I. Facts relating to Optical Science. No. III. Bi/ H. F. Talbot, 

 Esq., F.ii.S.* 



Optical Properties of the Iodide of Mercury. 



CHEMICAL writers have observed thai this substance ex- 

 hibits remarkable changes of colour. , It is oi'ange-red 

 when cold, but becomes yellow when moderately heated. As 

 it grows cold again, the red tint reappears. 



Sometimes, however, the yellow exhibits more permanency, 

 as has been remarked by Dr. Inglis in his Essay on Iodine f- 

 He says " that the yellow crystals of the biniodide retain 

 that colourybr a considerable time, unless suddenly cooled or 

 agitated, when the characteristic crimson tint of the biniodide 

 again appears." 



Wishing to examine into the cause of these facts, I placed 

 a small portion of the red iodide between two plates of glass, 

 and warmed it over a spirit-lamp. It immediately sublimed 

 into a yellow powder composed of minute crystals. As it 

 cooled, blood-red spots appeared upon the surface of the yel- 

 low mass and gradually spread themselves over the whole, 

 with the exception of some portions around the circumference 

 which usually remained yellow. When examined with a mi- 

 croscope this powder presented the curious appearance of 



• Communicated by the Author. Nos. I. and II. of these " Facts" will 

 be found in Loud. ;ind Kdinb. Piiil. Ma;;., vol. iv. pp. \V2, 389. 



f See the Number for January last, (vol. viii.) p. 18. 

 Third Scries. Vol.9. No, 51. Jm/// 1836. B 



