[ 336 ] 



LVII. On the Iodide of Mercury. By H. F. Talbot, Esq.. 



F.R.S. 

 To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



YOUR Number for last September contains a paper by Mr. 

 Warington " On the Change of Colour in the Biniodide 

 of Mercury." Permit me to observe, that the facts contained 

 in the first part of that paper were long ago discovered and 

 published by myself, in your Journal (S. 3. vol. ix. p. 2*). 



As I do not wish to be deprived of the discovery of one of 

 the most curious phaenomena in optics, I beg leave to draw 

 Mr. Warington's attention to that paper, and briefly to re- 

 capitulate its contents. 



In that memoir I have shown, — 



1. That when iodide of mercury is sublimed between two 

 plates of glass nearly in contact with each other, it cools in 

 the form of thin rhombic plates of a pale yellow colour. 



2. These often retain their colour when cold, if left undis- 

 turbed. 



3. But if such a crystal is disturbed, as for example, by 

 touching it with a needle at any point of its surface, it in- 

 stantly turns scarlet at the point touched, and the scarlet co- 

 lour is rapidly propagated over the whole crystal. I showed 

 this experiment to Sir David Brewster in the year 1836, and 

 I have no doubt he remembers it well, as he expressed great 

 admiration of the beauty of the phenomenon. The crystal 

 was touched with the needle while under examination with a 

 powerful microscope. 



4. The crystal moves and is spontaneously agitated during 

 the time it is changing colour. 



5. During the progress of this change, the scarlet portion 

 remains bounded by straight lines, very well defined, and par- 

 allel to the edges of the rhombic crystal. 



6. I thence drew the conclusion, that the change of colour 

 was caused by the displacement of the rows of molecules or 

 laminae of the crystal. This I think will be admitted to 

 be the true explanation ; and it was one which h ad not been 

 previously suggested. I added, that I thought this phenome- 

 non " the most evident proof we yet possessed of the dependency 

 of colour upon internal molecular arrangement" 



7. I also remarked that these little rhombic crystals were 

 very fine objects for the polarizing microscope. The expres- 

 sions of Mr. Warington, that the crystals " in the dark f eld 

 had the appearance of the most splendid gems" have recalled 



[• On inserting Mr. Warington's paper we referred to Mr.Talbot's pre- 

 vious experiments, as stated by him in Phil. Mag. — Edit.] 



