614 SIR DAVID BREWSTER ON CIRCULAR CRYSTALS. 
18, have placed themselves in these positions after the interior crystal has been 
formed ; that is, they are not increments deposited by the solution, but have been 
formed at a distance from the crystal, and carried to their new position. This is 
proved by the fact that sometimes a mass of them surround several of the square 
crystals, while individual ones take their place at random upon the face of the 
square. When the crystals are deposited from a strong solution, the square ones 
become almost opaque, and the irregular ones highly coloured, and of exceedingly 
various shapes. I have not been able to obtain any square crystals of the disul- 
phate of mercury from its solution in muriatic acid.* 
6. Parmeline.—This substance, dissolved in water, has a tendency to give 
circular crystals. In alcohol it gives very fine ones, producing, when small, 
beautiful halos like oil of mace, with blue light in their centre. 
7. Asparagine and Salicine mixed.—After standing several months, this mixed 
salt produced small circular crystals, apparently of asparagine.. These crystals 
gave brilliant halos of red and green light, of such a diameter that the individuals 
were only =zgoth of an inch in diameter. Among these small crystals were placed 
large circular discs, with curved sectors and black crosses, which gave them the 
appearance of the corolla of a flower with party-coloured petals. 
8. Palmic Acid.—This substance, when melted by heat, gives very fine negative 
circular crystals like those in the hexagonal mosaic of manna. Theinsulated 
discs have a rim sometimes divided by broad black bands, where the substance 
was too thin to polarise the light. When the rim is broad and single, it is com- 
posed of narrow luminous sectors, radiating from points in the circumference of 
the disc. The rims are sometimes of a different colour from the principal sectors, 
and the latter are often subdivided by numbers of black and equidistant concentric 
circles. 
9. Nitrate of Uranium.—This salt gives fine negative circular crystals in 
water, alcohol, and ether. The crystals formed in the alcoholic solution deli- 
quesced in an instant, forming hemispherical bells, which polarised the light by 
oblique refraction, giving four luminous sectors, and a black cross very wide at 
the centre, like the sectors and cross produced by the hemispherical cups of de- 
* In making these observations, and on many other occasions, I have felt the great inconvenience 
of the present, and in general, perhaps the best, arrangement of the compound microscope. High 
powers being always obtained by object-glasses of short focal length, it is almost impossible, in 
transparent structures, to develope them, when they consist of lines or parts of different thickness. 
Vision is destroyed by the refractions and diffractions of the intromitted light. The only remedy for 
this is to use }-inch, or even 1 or 2-inch object-glasses, and obtain the power that is required at the 
eye-piece, by means of grooved and other lenses of diamond, garnet, &c. 
