Colour in the Biniodide of Mercury. 193 



If this precipitated biniodide, in the dry state, be subjected 

 to the action of heat, it becomes of a bright pale yellow colour, 

 fuses into a deep amber-coloured fluid, and gives off a vapour 

 which condenses in the form of rhombic plates of the same 

 bright yellow;- these crystals, by any mechanical disturbance, 

 arising from the unequal contraction of their molecules in cool- 

 ing, from varying thickness in different parts of the same cry- 

 stal, or from partial disintegration, return again to the origi- 

 nal scarlet colour of the precipitate, the change commencing, 

 in the latter case, from the point ruptured, and spreading over 

 the whole of the crystalline mass; they may however be fre- 

 quently preserved in the yellow state for a great length of time, 

 if sublimed slowly and not exposed to the contact of other sub- 

 stances, which is readily effected by conducting the sublima- 

 tion in closed vessels, and allowing the crystals to remain in 

 them undisturbed. 



The resumption of the scarlet colour has been attributed to 

 an alteration in the molecular arrangement of the crystals, and 

 it was with the view of clearly ascertaining this point that the 

 following microscopic investigations were undertaken. 



When a quantity of the precipitated biniodide is sublimed, 

 the resulting crystals are very complicated in their structure, 

 consisting of a number of rhombic plates, of varying size, su- 

 perposed, sometimes overlapping each other and causing con- 

 siderable variableness in their thickness, but generally leaving 

 the extreme angle and the two lateral edges clear and well- 

 defined ; the annexed sketch, taken by the camera lucida from 

 the field of view of the microscope, will give a better idea of 

 their character. The length of these crystals was about *01 5 

 of an inch in length. On cooling, the first change that is ob- 

 served is usually a scarlet marking, commencing at the ex- 

 treme angle and extending gradually inwards, always retain- 

 ing a perfectly well-defined line in its progress; when this 

 change has reached as far as the line ab, fig. 1, the scarlet 

 line will suddenly shoot along one of the lateral edges, as shown 

 at c d, and instantly the whole mass is converted, in a most rapid 

 and confused manner, which the eye in vain endeavours to fol- 

 low, to the scarlet colour, the crystal being frequently, if de- 

 tached, twisted and contorted during the transition. 



In order to obtain these crystals in a more defined and 

 clearly developed form, a small glass cell was constructed of two 

 slips of window-glass, leaving a space of about the thickness of 

 cartridge paper between the upper and under plates, in which 

 the sublimations could be readily conducted, and the whole of 

 the subsequent changes at once submitted to the microscope ; 

 Phil. Mag. S. 3. Vol. 21. No. 137. Sept. 18*2. O 



