On the Intimate Structure of Crystals. 



273 



such as cuprous oxide (Cu 2 0), or fluorspar (CaF 2 ), possess an octa- 

 hedral cleavage. Let us for the moment follow the hint given by 

 Haiiy, and regard the cubic crystals with cubic cleavage as reduced 

 by repeated cleavage to a primitive cubelet. How in the case of a 

 diatomic compound composed of atoms of different relative sizes, as, 

 say in common table salt, can the molecules be arranged together so 

 as to produce this cubelet ? 



FIG. 1. 



The only satisfactory way that I can discover is to place one mole- 

 cule (NaCl) by the side of another (ClJSTa), as in the diagram (fig. 1), 

 so that tangent planes to the pairs of atoms may form a square 

 prism, and then to rest on the top of this two more molecules, simi- 

 larly put together, the chlorine of the upper group resting on the 

 sodium of the lower, and the sodium of the upper on the chlorine 

 of the lower ; common tangent planes to the exterior of the spheres 

 taken in fours will be found to form a cube. 



This is a simple and possible arrangement of the molecules of 

 common salt, but whether it alone and not some other is the actually 

 existent arrangement is a question for argument. 



Ever since Kopp formulated his famous laws, the subject of 

 atomic volumes has possessed great interest for the chemist, though 

 the discrepancies which too frequently impair the correspondences 

 discovered by Kopp have evoked a general feeling of mistrust. 



Nevertheless I am persuaded that in atomic volumes we possess the 

 touchstone by which our conjectural arrangements may be brought 

 to the test. The study of atomic volumes and crystalline structure 

 cast mutual light upon each other. 



Kopp obtained his volumes by dividing the molecular weight (m) 

 of a substance by its density (d), thus m/d = v. Evidently if 

 atoms have the form of solids of revolution, this volume is a lump 

 quantity, including not only the volume of the atoms, but of the 

 interstices between them. We shall therefore speak of it as the 

 gross volume (V), in contradistinction to the true relative volume (v) 

 of the atom. 



The shortest way of explaining our method of investigation will 

 be to describe at once the process as applied to the haloid compounds 

 of the alkali metals. Let us commence with common table salt. Its 



