THE DISINTEGRATED SURFACES OF CRYSTALS. 175 



able changes upon the figures which they produced. The changes take place prin- 

 cipally upon the central parts of the figure, as shewn in Fig. 38, which represents 

 one of the forms which a solution of nitre gave to the figure produced by alum ; 

 but in other cases the whole figure suffers a change. A crystal of sulphate of po- 

 tash, which gave the hexangular radiations already described, produced the same 

 figure, with twice the angular magnitude, when dipped for a few seconds in a sa- 

 turated solution of nitre. 



In consequence of having observed that the natural cleavage planes of crys- 

 tals gave indications of regular optical figures, similar to those produced by solu- 

 tion, I Avas led to make some experiments on the effects of mechanical abrasion, 

 as produced by coarse sandstone, or by the action of a rasp or large-toothed file. 

 Surfaces thus torn up produced, in a rude manner, the optical figure given by so- 

 lution ; but what was very remarkable, the figure had a different position, or had 

 the position which solution would have developed on the opposite face. This is 

 also true of the figures produced by natural cleavage planes, in which the sepa- 

 rating surfaces have been slightly torn up. 



It is scarcely necessary to observe, that the power of producing the optical 

 figures described in this paper may be communicated to wax or isinglass, &c. The 

 impressions on isinglass enable us to see the figure by transmitted light, and to 

 observe its form and dimensions with greater accuracy. 



Allerly, February 1. 1837. 



