THE DISINTEGRATED SURFACES OF CRYSTALS. 17| 



After the specimen had been exposed for some time to the action of boiling 

 muriatic acid, the face which had given Fig. 19, now exhibited the remarkable 

 phenomena shewn in Fig. 20. The nebulosity had almost disappeared from the 

 interior of the square, and collected, as it were, in its centre. The brightest parts 

 of the figure were the curved masses at the angles, the middle parts of the sides 

 of the figure being exceedingly faint. 



From the tessular I proceeded to the rhombohedral system of crystallization, 

 and I employed calcareous spar and sulphate of potash in the inquiry. 



Having immersed a rhomb of calcareous spar in dilute niti^ic acid, four parts 

 of water being added to one of acid, I observed the reflected figure from all the 

 faces of the rhombohedron, to have the form shewn in Fig. 21. The obtuse angle 

 of the crystal was in the direction CE, and the angle ACB was greater than 120°. 

 As the obtuse angle of the opposite face has an opposite direction, the figure which 

 it gives by reflexion is the inverse of Fig. 21, so that, by looking through the 

 parallel faces, we obtain a figure with six luminous radiations. By varying the 

 strength of the acid, the time of its action, and taking the surfaces of different 

 crystals, the figure undergoes remarkable changes; but though two individual 

 figures often occur between which no similarity exists, yet, by observing the tran- 

 sitions of a considerable number, we may trace the family likeness through them aU. 



The thin web of light AEB, BD, and DA, appear at an early stage of the 

 action, but it is often wanting between A and B, and by continuing the action, a 

 radiation often appears at F, sometimes united, and sometimes not, with the 

 centre C. The radiation CD sometimes expands suddenly below C, into a diver- 

 ging brush of light, and in other cases it is often wholly wanting, as well as the 

 triangular luminous centre C. In this case we have only two luminous brushes, 

 A, B, with a small central image at C, A and B being sometimes joined by bright 

 light, and sometimes by a small arch of nebulous light, the centre of which was 

 ate. 



On the faces of three different crystals, a figure with^w radiations, diverging 

 at unequal angles, was produced. Two of these were the radiations A, B, the 

 third was the brush developed at E, and the fourth and fifth were formed by the 

 division of CD into two branches. Sometimes the whole of the central part of 

 this five-rayed figure was wanting, leaving the expanded part of the raditition in 

 the circumference of a sort of oval ring, which was sometimes luminous through- 

 out, but studded with the five brushes of stronger light. 



When the solvent was pretty strong muriatic acid and water, the figures 

 have often a great similarity to those already described, but in some cases they 

 have the form of luminous shields of a triangular foiTQ, as shewn in Fig. 22. The 

 place of the central image is at C ; the brightest part of the figure is at E, with a 

 reddish margin, and the next l)rightest at A and B. In other crystals the lights 



