174 SIR DAVID BREWSTER ON THE OPTICAL FIGURES PRODUCED BY 



sulphate of iron, and sulphate of copper, but, though I have dehneated many of the 

 figures which they produce, and though some of them have considerable interest, 

 I am not able to present the details in the foi-m which I could wish. I expected 

 to have been able to obtain interesting and definite results by subjecting the faces 

 of a large class of minerals to the action of fluoric acid ; but, in so far as my ex- 

 periments went, I was disappointed. Dr Fj^e, many years ago, exposed several 

 crystals of quartz and amethyst, which I sent him for this purpose, to the action 

 of fluoric acid, but the disintegration of the surfaces Avas such that they would 

 not reflect any light at all. I have no doubt, however, that, by weakening the 

 action and carrying it on very slowly, the desired effect will be produced. 



During the preceding experiments I was led to observe, that different solvents 

 had a tendency to produce different figures, and I confirmed the truth; of the ob- 

 servation by many experiments. When muriatic acid, for example, acts upon 

 alum, it produces a figm*e with six radiations, not unlike those of sulphate of po- 

 tash, and, by continuing the action, the central image vanishes. If in this state 

 we immerse it in water, three of the radiations vanish, and it assumes the usual 

 form. When again immersed in muriatic acid the six images reappear. Diluted 

 nitric acid has the same effect as muriatic acid ; but diluted sulphuric acid gives 

 such a form to the radiations, that their extremities are included within an equi- 

 lateral triangle, the larger radiations pointing to the three angles, and the shorter 

 ones to the three sides. 



Diluted alcohol, though it acts feebly upon alum, produces a figure different 

 from water and the acids. It gives a figure with three short radiations ; and, by 

 farther dilution, the figure undergoes changes which give it a greater resemblance 

 to the aqueous figure. 



In order to retard or diminish the action of solvents upon highly soluble crys- 

 tals, I conceived the idea of immersing them in solutions of the crystal of different 

 degrees of strength. In making this experiment on alum, I took a crystal ^vhich 

 gave the figure shewn in Fig. 8, and, having immersed it in a saturated solution 

 of alum for a single instant, I found that it had, as it were, seized the particles of 

 alum in the solution, and replaced them in their proper position on the disinte- 

 grated face. By subsequent immersion the face repassed through aU the stages 

 at which it produced the phenomena shewn in Fig. 7, and finally became perfect, 

 reflecting a single image of the candle. The singular fact in this experiment is, 

 the inconceivable i^apidity with fvhich the particles in the solution fly into their 

 proper places upon the disintegrated surface, and become a permanent poi'tion of the 

 solid crystal. 



In repeating and varying these experiments, I observed a number of curious 

 facts, which it would be out of place here to describe. I immersed crystals of 

 alum in satm*ated solutions of nitre and other salts, and observed many remark- 



