OF ARTS AND SCIENCKS. 



247 



the same triangular outline to each. It appeared as if the crystal had 

 been in a melted state, and that, soon after the outside had congealed, 

 the inner and yet fluid portion, or a part of it, had run out, leaving the 

 surrounding consolidated edge just referred to. Appearances quite simi- 

 lar may sometimes be observed among artificial crystals, as for instance 

 alum, and, more strikingly, metallic lead (which takes the form of the 

 octahedron and has become partially desulphurized), in cases where the 

 metal was allowed to flow off" slowly, just as the outer crust had formed 

 over the surface of the crystals. The large crystal presents only 

 one half of the octahedron, its base blending with the massive gold, 

 or only indicating the incipient planes of the lower pyramid. Three of 

 its planes are perfectly smooth, excepting along the edges, which are 

 prominendy marked by the same projecting border or ridge observed 

 on the smaller crystal. This border may have been produced in the 

 same manner by the shrinking away of the metal, or it may be the 

 result of that kind of crystallization which is dependent on a greater 

 intensity of molecular attraction in one direction or axis than another. 

 It would seem in this case as if the molecules arrived at the points of 

 contact along the edges of the crystal faster than they could be ap- 

 propriated, and thus have accumulated in these little ridges. This 

 peculiarity is not confined to the large crystals, for it is observed even 

 among the smallest. In one instance, as shown on a crystal of a 

 half-inch in diameter, there had been produced a double series of 

 these parallel ridges, extending around the edges of one of the planes 

 of the octahedron, the inner ridge representing, apparently, the com- 

 mencement of another crystalline face within the cavity of the larger 



one.* 



" The great size of these crystals, and the fact that some of the cavi- 



* The two large crystals above described were obtained from the very choice 

 and beautiful collection of specimens, made with great care, and at no small ex- 

 pense, by Mr. Piatt. This gentleman, during a most prosperous residence of two 

 years in San Francisco, and while occupying a situation which brought him into 

 daily and almost hourly contact with persons returning from the mines, has 

 evinced his good taste by purchasing the most interesting specimens obtained by 

 them. He has consequently been rewarded by the finest amateur collection hith- 

 erto brought from California. It comprises a great variety of ramified, arborescent, 

 dendritic, and other imitative forms, here and there showing crystalline faces, all 

 of them being sometimes most fantastically joined together in the same specimen. 

 He informs me that, in obtaining this collection, he had examined gold of the 

 value of more than four millions of dollars. 



