196 Mr.Warington on Change of Colour in Biniodide of 'Mercury. 



sublimation, formed in a curved line across the field of view 

 and extended slowly downwards ; by the strong transmitted 

 light these minute crystals appeared colourless ; but when 

 viewed by reflected light, the pale yellow colour was readily 

 apparent. After a short interval a very extraordinary change 

 commenced; the crystals, which had been perfectly sharp and 

 well-defined, became ragged at their edges, as though some 

 dissolving action were going on, gradually decreased in size, 

 and at last disappeared altogether; but as this act of solution 

 progressed, numbers of red crystals made their appearance, 

 forming across the field and following at a regular distance 

 the yellow crystals as they disappeared, and occupying their 

 place. These red crystals, which appear to be formed by the 

 disintegration through the medium of solution, if I may be 

 allowed the expression, from those first produced, had the 

 form of the octohedron with the square base, exactly similar 

 to those procured by careful sublimation at a low heat, only 

 modified in the most beautiful manner. Some few of these 

 are sketched in the forms, «,&,c, 

 d, e,f, g, //, fig. 4. When either 

 the salt of mercury or the iodide 

 of potassium, employed in the 

 production of the biniodide of ( 

 mercury, was in excess, another 

 curious act of disintegration took 

 place ; the red crystals in fig. 4- 

 were slowly dissolved, aproperty 

 mentioned in the first part of this ft, 

 paper, the first act of solution y i \ 



commencing apparently by the k f^ l k _J 



disjunction of the crystals «, b, 



c, f g, h, at the lines of marking, these lines being at first 

 bright red, and gradually deepening in colour when the 

 act of solution commenced, and at last perfect separation 

 taking place, so that the light could be seen between the 

 compartments. At times the field would become dry from 

 evaporation, and some of the yellow rhombic crystals which 

 had not been dissolved, prior to the formation of the octohe- 

 dra with the square base, were observed with scarlet lines on 

 them similar to the first act of transition in the sublimed cry- 

 stals, as shown at g 1 and 2 in fig. 2. 



By polarized light the appearances now described were 

 beautiful beyond all description, the yellow crystals present- 

 ing the most superb and brilliant colours, varying in hue with 

 the varied thickness of the crystalline plate, and in the dark 

 field having the appearance of the most splendid gems the 



