194< Mr. Warington on the Change of 



by this means beautifully well-defined and perfect crystals 



Fig. 1. 



were obtained, having the form of right rhombic prisms, as in 



Fig. 2. 



the accompanying outlines, 

 fig. 2, a and b. The follow- 

 ing interesting phagnomena 

 were then observed : a de- 

 fined scarlet line of varying 

 breadth would shoot across 

 the crystal, as at 1 . c, d, e,f, 

 fig. 2, and then gradually 

 spreadthroughoutthewhole 

 of its structure, keeping a 

 straight and well-defined 

 line in its onward progress, 



until the whole had undergone the change of colour. Nos. 2, 3, 

 4f, 5 in e, and No. 2 in f, are the stages which the transition had 

 reached at intervals of observation; in many cases, after the 

 crystal has undergone this metamorphosis, two angles can be 

 distinctly seen, as at e, fig. 1, and at times two edges are visible, 

 as at c 6 and d 6, fig. 2. This observation must of course de- 

 pend entirely on the position of the crystal to the eye of the 

 observer. 



These phaenomena prove, I consider, in the most perfect 

 manner, that the change in the colour of this compound arises 



