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V 



D 



Colour in the Biniodide of Mercury. 195 



from the plates of the crystal having been separated from each 

 other, by the means alluded to, in the direction of their clea- 

 vages ; and in further confirmation of this view, the lamina? 

 so separated may, by the sudden application of heat, be again 

 fused together, and the yellow colour reproduced without ma- 

 terially altering the dimensions of the crystal, a slight round- 

 ing of the edges from partial sublimation being the only other 

 concomitant. 



When the temperature is raised slowly and the sublimation 

 conducted with great care, a verylarge proportion of red cry- 

 stals, having a totally different form, are obtained, the octahe- 

 dron with the square base, YW. 3. 

 as shown fig. 3, a, b, c, d, e. 

 If, however, the heat is 

 quickly raised, the whole 

 mass of the sublimed cry- 

 stals are yellow and of the 

 rhombic form. It is evident 

 from these facts, that the 

 biniodide of mercury has 

 two vapours which are given off at different temperatures, 

 and also that it is dimorphous, which facts have been sub- 

 stantiated by some experiments of M. Frankenheim, who has 

 carefully examined this part of the subject. 



From the circumstance that the first effect which occurs in 

 the process for preparing this iodide by precipitation is the pro- 

 duction of a yellow powder which passes rapidly through the 

 orange colour to a scarlet, I was induced to submit this phe- 

 nomenon also to the test of microscopic examination, and with 

 this valuable instrument of research, results were exhibited 

 which could not have been anticipated. As I expected, the 

 precipitate was in small crystalline grains, and the first step of 

 the investigation was to effect its formation in the field of view 

 of the microscope, so as to observe, directly as they occurred, 

 the transitions of colour which have been alluded to, and this 

 was effected by the following means : — A slip of common win- 

 dow-glass, about three inches long by one and a half wide, and 

 having a very narrow slip attached on one of its edges, so as 

 to act as a ledge, was taken, and a drop of the salt of mercury 

 employed placed on it ; this was then covered with a small 

 piece of extremely thin glass, about one inch long by half an 

 inch wide, and the whole carefully adjusted to focus in the 

 field of the instrument; the iodide of potassium was then in- 

 troduced by capillary attraction between the glasses. The 

 instant the solutions came in contact, a myriad of pale-yellow 

 crystals, having the same rhombic form as those obtained by 



02 



