the Disintegrated Surfaces of Crystals. 25 



figure shown in PL IV. fig. 23, the letters having the same indi- 

 cations as in the preceding figures. 



I now proceeded to apply the solvents to the summit planes 

 of the prism. My first experiment was made on an artificial 

 face perpendicular to the axis. By the action of vinegar it gave 

 the figure shown in fig. 24_, which consists of three radiations, 

 inclined 120° to each other, having its centre sometimes dark, 

 and sometimes occupied with a small image. The rudiments of 

 other three radiations, inclined 60° to the former, are distinctly 

 visible; and beside a luminous circle circumscribing the whole, 

 there are three non-concentric circular arches, similar to those 

 seen in fluor-spar. 



The very same figure, with the exception of the circle and the 

 circular arches, was obtained from the action of dilute muriatic 

 acid on the natural faces of the chaux carbonatee hasee of Hauy. 



Having ground and repolished the artificial summit which 

 exhibited fig. 24, I exposed it to the action of dilute muriatic 

 acid, when I was surprised to see it produce the strange figure 

 shown in fig. 25. Although the symmetry of the figure is hos- 

 tile to the idea that its shape might have been partly the effect 

 of accident, yet I found it unaltered by repolishing, and again 

 disintegrating the surface, and what is still more decisive, I 

 obtained the very same effect twice from another crystal of cal- 

 careous spar*. 



By placing the crystal which gave this remarkable figure in a 

 stronger acid solution, it gave on both its faces the figure in 

 fig. 26, the light of which is strongest in the circular arches. 

 By continuing the action of the same acid, the three inclosed 

 radiations disappear entirely, and what is still more singular, 

 they reappeared by a further continuance of the action. The 

 action being prolonged tCey again disappeared, the circular 

 arches grew wider and more confused, till they filled up the space 

 which they at first inclosed. 



Another crystal of spar exhibited the very same series of suc- 

 cessive changes which I have now described. 



I now reground and polished the faces of both these speci- 

 mens. When they were plunged into strong dilute acid, their 

 disintegrated surfaces produced no figure ; but by increasing the 

 strength of the solution, the figures were developed as formerly. 



In order to observe the effect produced upon faces that were 

 not coincident either with the primary or secondary faces of the 

 crystals, I ground down one of the acute solid angles, and 

 replaced it by a plane inclined 71° .to a face of the rhomb, the 

 common section being parallel to the long diagonal of this last 

 face. After being immersed in dilute nitric acid, it gave the 



* The brightest part of the figure was ah^ the part above a being faint. 



