447 



of the same mass present also the square arrangement, and accord- 

 ingly certain portions of it exhibit the octohedral group. 



The author remarks, in support of this theory, that a large pro- 

 portion of those substances which assume the octohedral form, are 

 considered by chemists as simple bodies, and are therefore more likely 

 to have the simple form of spheres than such as consist of more than 

 one element. Since the supposition of spherical particles appeared 

 to him to afford so satisfactory an explanation of an acknowledged 

 difficulty in crystallography, he was led to consider what other forms 

 would result from the union of solids most nearly allied to spheres ; 

 and he observed that obtuse rhomboids, like those of carbonate of 

 lime and other substances, would be formed by the union of oblate 

 spheroids, as indeed Huyghens had long since observed ; and that by 

 the union of oblong spheroids, the natural result would be triangular 

 and hexangular prisms, as are found in beryl and phosphate of lime. 



But the most singular arrangement noticed, is that which affords 

 an explanation of the origin of cubes in crystallography. These, he 

 supposes, may consist of spherical particles, of two different kinds, 

 regularly intermixed in equal numbers (in conformity to the most re- 

 cent views of binary combination in chemistry) ; for these, he ob- 

 serves, will not tend, as before, to the octohedral arrangement, but 

 will be perfectly in equilibrio when every group of eight balls com- 

 poses a cube, according to the most obvious course of alternation of 

 the two elements. For in that case all similar balls will be equi- 

 distant from each other, and will also be equally distant from all ad- 

 jacent balls of the opposite denomination. 



In a note are subjoined some observations on a theory of crystalli- 

 zation proposed by M. Prechtl, who imagines that a mass of soft 

 spheres may all be compressed into tetrahedra, which is demonstrably 

 impossible. That by another degree of softness or of attraction, 

 spheres, each surrounded by five others, may be compressed into tri- 

 angular prisms, without regard to the different degree of compression 

 that must take place in the direction of the axis ; that other spheres 

 again less compressible than before, and consequently surrounded by 

 as many as six others, may be formed into cubes, which indeed is 

 admitted to be a very possible supposition. 



It is observed, however, that M. Prechtl denied that a sphere can 

 be surrounded by more than six, although, in fact, the most probable 

 supposition is, that each soft sphere would be surrounded by twice 

 that number, and would form a mass of regular dodecahedra. 



On a Substance from the Elm Tree, called Ulmin. By James Smith- 

 son, Esq. F.R.S. Read December 10, 1812. [Phil. Trans. 1813, 

 p. 64.] 



The substance here examined by the author, we are told, was first 

 made known by the celebrated Klaproth. It has been ranked as a 

 distinct principle, soluble in water, but insoluble in alcohol or ether, 

 and convertible, by the action of nitric or oxymuriatic acids, into a 



