I4b ATTRACTION. 



" The piece of brass, in which the pivot is fastened, slides in a slit 

 in each leg, so as to permit them to be made of the most suitable 

 length, on the side on which the crystal is applied." 



The reflective Goniometer of Dr. Wollaston, depends upon the 

 reflection of the rays of light from the brilliant surfaces of contigu- 

 ous crystalline plates, uncovered by cleavage, or of natural surfaces. 

 The pieces or crystals to be examined are fixed upon an axis whose 

 revolution carries around a graduated wheel, which measures the an- 

 gle contained between two contiguous surfaces, when they have ar- 

 rived successively in the position to reflect an image of the bar of a 

 window or of some other definite line.* This instrument is much 

 more accurate than that of Carangeau, used by Hauy, (See the fig- 

 ure above,) and has corrected a number of errors, some of which 

 were important. 



Mr. Daniell has contrived a method of discovering the structure of 

 crystals by solution. In a mass of alum lying in water, there will be 

 discovered, after some -time, upon its lower part in high relief, both 

 octahedral forms and sections of octahedra. Borax gives similar 

 results. Even shapeless metals, which a peculiar tendency to crys- 

 tallization, will reveal their crystalline forms by the action of acid sol- 

 vents; bismuth exhibiting with dilute nitric acid, cubes, antimony, 

 rhomboidal plates, and nickel, regular tetrahedra.f 



Very different views of crystallization are taken by more recent 

 authors, among whom Mr. Brooke f and Professor Mohs are the 

 most distinguished. Crystalline forms that have an intimate connex- 

 ion with each other, are considered as forming certain natural groups 

 or systems of crystallization. They are called, the tessular system 

 which comprehends the cube, the tetrahedron, the regular octahe- 

 dron, the rhombic dodecahedron, &c. ; the pyramidical system, con- 

 taining the octahedron with a square base and the right square prism ; 

 the prismatic system including the rectangular and rhombic octahe- 

 dron, and the right rectangular and right rhombic prisms ; the hemi- 

 prismatic system, embracing the right rhomboidal and the oblique 

 rhombic prisms ; the tetarto-prismatic system containing the oblique 

 rhomboidal prism, and the rhombohedral system comprehending the 

 rhombohedron and the regular hexagonal prism. || 



This complex system seems to present no advantage to compen- 

 sate for the absence of the simplicity and perspicuity which charac- 

 terizes the system of Hauy. 



* A more particular description with a plate maybe found in Phillips' Mineralogy 



t English Jour. Sci Vol. I. p. 24. 



t Familiar Introduction to Crystallography 



Treatise on Mineralogy, translated by Mr. Haidinger. 



it Turner, 2d Ed. p. 555. 



