Proceedings of the Royal Society of London/Volume 2/On the Effects of simple Pressure in producing that Species of Crystallization which forms two oppositely polarized Images, and exhibits the complementary Colours by polarized Light
The author having, in former experiments on the depolarization of light by a mixture of resin and bees' wax, had reason to suppose that the effects were modified by pressure, now examines the effects of pressure on a class of substances which, from their elasticity, will restore themselves after removal of the force applied, and thence admit of greater variety in the repetition of the experiments. By employing animal jellies, he had an opportunity of giving them any degree of tenacity that might be wished.
A small cylinder of jelly being placed upright between two plates of glass, had at first no power of depolarization. By gradual drying at its circumference, it soon began to depolarize at that part; and as it became thereby more dense than at its centre, it had the power of a concave lens. At the end of three weeks it seemed dried to the centre, and had then lost both these properties; but by forcible pressure, which it could now bear without injury, it depolarized completely during the continuance of the pressure; but upon its removal, says Dr. Brewster, it resumed its uncrystallized state.
The author next employed isinglass jelly, brought nearly to the consistence of caoutchouc, which, after standing one day, had acquired the depolarizing power even when cut into thin slices; and when forcibly compressed between two plates of glass, it also exhibited beautiful colours, that were complementary to each other in the two images of a candle seen through it, by means of a prism of Iceland spar; and when the pressure was removed, these complementary tints disappeared.
Inasmuch as these colours might be supposed owing to the thinness to which the plate of jelly was reduced by pressure, Dr. Brewster cut the cake to the same thinness which it had possessed while under compression, but without any production of colours till pressure was again applied.
In the author's concluding experiment, he formed one twentieth of an inch thick of the same jelly, by melting it between two plates of glass. When merely consolidated by cooling, this had no power of depolarization; but by pressure it instantly restored the evanescent image, and exhibited, as in the former cases, the complementary colours, showing, says the author, that pressure communicates a modification of structure correspondent to that of crystallized minerals.