492 Sir D. Brewster on the Phenomenon of Luminous Kings 



to me. In cutting the crystal Mr. Sanderson found that one 

 end of it was foul, and produced a luminous ring round a 

 candle. I have examined it with particular care, and have 

 found that the rings are produced by tubular cavities, and that 

 several of these cavities contain the two new fluids, while others 

 have contained them, and have been emptied of their contents 

 by being cut through by the lapidary at one or both ends. 

 These tubes are accurately parallel to the axis of the six-sided 

 prism; several of them have irregular shapes like some of those 

 which I have figured in my papers On the two New Fluids in 

 Minerals*. In the opened tubes their interior is covered with 

 an indurated crust which the fluid has left behind, while in 

 others the fluid is distinctly visible with high magnifying 

 powers. In two or three parts of the specimen there are 

 spaces or strata of fluid cavities perpendicular to the axis of 

 the prism, and of a hexagonal form. Hundreds of these ca- 

 vities are like mathematical points, which the highest magni- 

 fying powers are unable to resolve, but many of them are suf- 

 ficiently large to exhibit clearly the two new and immiscible 

 fluids which exist in the same cavity, and the great expansion 

 by heat of the volatile fluid. In the many hundred specimens 

 of topaz and other minerals in which I have had occasion to 

 examine the physical character of these fluids, the denser of 

 the two which occupies the angles and narrow portions of 

 cavities is much smaller in quantity than the volatile fluid, 

 which flies off in a gaseous form when the cavities are opened 

 or burst by heat. It is quite otherwise, however, in the beryl 

 cavities. The volatile fluid, with its apparent air-bubble or 

 vacuity, floats in a small quantity of the denser fluid, and ex- 

 pands so as to fill that vacuity at a temperature of 60°. In 

 several of these cavities there are minute crystals, but I have 

 not ventured to apply such a degree of heat as to determine 

 whether or not they will melt and recrystallize like those in 

 the topaz cavities. 



As the tubular cavities in this remarkable specimen of beryl 

 are larger than those in calcareous spar, the luminous ring which 

 they produce is not such a smooth and regular band of light as 

 it is in the latter mineral. The ring is composed of a number 

 of radial lines closely packed together, and in particular lights 

 it derives from this structure a great degree of beauty. As 

 there is no unbalanced dispersion, the ring is perfectly white; 

 and at three points of its circumference 120° distant, there is 

 an image of the luminous body. The ring of course does not 

 consist of polarized light like the rings in calcareous spar, but 

 is composed of two rings oppositely polarized. If the cavities 

 * Edinburgh Transactions, 1823, vol. x. p. 1, 407- 



