174 SIK DAVID BREWSTER ON THE OPTICAL FIGURES PRODUCED UV 



sulphate of iron, and sulphate of copper, but, though I have delineated 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 fonn 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- 

 ])eriments went, I was disappointed. Dr Fyfe, many years ago, exposed several 

 crystals of quartz and amethyst, which 1 sent him for this purpose, to the action 

 of fluoric acid, but the disintegi-ation of the surfaces was 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 eifect wiU be produced. 



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

 had a tendency to produce different figm-es, and I confirmed the trutli; of the ob- 

 servation by manj' experhnents. When mm-iatic acid, for example, acts upon 

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

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

 Ave immerse it in water, tlu-ee of the radiations vanish, and it assumes the usual 

 lorm. When again unmersed in mmiatic 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 tlu'ee sides. 



Diluted alcohol, though it acts feebly upon ahim, produces a figure difierent 

 ti-om water and the acids. It gives a figm-e with three short radiations ; and, by 

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

 to the aqueous figm-e. 



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 experuuent on almn, I took a crystal ^hich 

 gave the figm-e shewm in Fig. 8, and, having immersed it in a saturated solution 

 of aliun 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 thi-ough aU the stages 

 at which it produced the phenomena she\STi in Fig. 7, and finally became pei-fect, 

 reflecting a single image of the candle. The siuf/iilar fact in this experiment is, 

 the inconceivable rapidity with which the particles in the solution fly into their 

 proper places upon the disintegrated surface, and become a permanent portion of the 

 solid crystal. 



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

 facts, which it would be out of place liere to describe. I immersed crj'stals of 

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



