CRYSTALLIZATIONS. 100 



ALUM. In drawing 162 are delineated several groups of the crystals of alum. 

 This substance is for the most part artificially produced,, and is seldom found in 

 a native state, though it occasionally appears as an efflorescence, and exists in 

 certain mineral waters in the East Indies. The primitive form of the crystals 

 of alum is an octahedron ; that is, a regular solid contained within eight equal 

 faces. The combinations of the single crystals are extremely rich, and the fig- 

 ures in the drawing but faintly represent the exquisite crystalline tissue which is 

 seen beneath the microscope emerging from the shapeless fluid ; every part sym- 

 metrically wrought with lines of fairy gems, replete with elegance and beauty. 

 The crystals form rapidly, and are beheld on the slide shooting forth into the 

 liquid film, in figures like those delineated at A, being arrow-headed and serra- 

 ted at the edges. Advancing side by side in parallel lines, they each spread 

 rapidly on either hand, throwing out lateral spurs at right angles to the main 

 crystals. From the edges of these secondary crystals, others in like manner dart 

 forth, all weaving and interlacing with each other, and forming, with other simi- 

 lar systems, one unbroken, glittering sheet. The advancing line of such a crys- 

 talline field, is displayed in figures 13, C, and D. Sometimes the main crystals, 

 as at C, slightly diverge from each other. An unusual configuration is de- 

 lineated at E, where two sets of crystals, bending in parallel curves, intersect 

 each other, and form, by their union, a light and graceful gothic arch. The 

 breadth of the larger crystals across their arrow-heads, , &, measures about one- 

 two hundredth of an inch. 



SALT, OR CHLORIDE OF SODIUM. In drawing 163, an assemblage of crystals of 

 common salt is delineated. The primitive form of this well-known substance is 

 a cube, and the crystalline structure of the fragments and masses in which it is 

 found, is seen at a casual glance with the unaided eye. A solution of salt, as it 

 crystallizes, is never seen spreading out into beautiful ramifications ; but as fast 

 as the fluid evaporates, the surface of the glass becomes studded all over with 

 minute and sparkling gems of salt. Eight-sided and twelve-sided figures are 

 likewise formed by the union of the primitive cubical crystals. Another variety 

 which is quite common, is that of a hollow rectangular pyramid. It begins its 

 formation at the surface of the fluid with a small cube, upon the upper edges of 

 which four rows of small cubes soon crystallize. To their upper and outer 

 edges other cubical crystals now attach themselves, and by degrees a hollow py- 

 ramidal structure is completed capped at the smaller extremity with the origi- 

 nal cube. In the drawing several cubical crystals are delineated, together with 

 many specimens of the hollow pyramid. The length of one of the sides of the 

 crystal A, is the two hundred and twenty-seventh part of an inch. 



SNOW. The snow-flake, which varies from more than an inch to seven-hun- 

 dredths of an inch in diameter, consists of an assemblage of exquisitly minuto 

 crystals ; and from its beautiful figures and rich diversity of forms, has ever ex- 

 cited the admiration of observers. 



