388 Prof. Norton on Molecular Physics. 



which crystallization occurs should become united with those 

 that take up their positions at the angles of the plane figure. 

 Thus two sets of four molecules in parallel planes may take up 

 positions of equilibrium at the eight angular points of a cube, 

 or six may form an octahedron, &c. The conditions that will 

 determine the figure of equilibrium assumed may be inferred 

 from the general considerations already presented (pp. 385 and 

 386). The compression of the molecular atmospheres in the 

 first plane of crystallization will tend to develope a third axis of 

 attraction perpendicular to this plane. Various systems of crys- 

 tallization are possible, since the particles on one side of the 

 first plane may take up positions of equilibrium perpendicularly 

 opposite to those that crystallize in this plane, or opposite the 

 intervals between parallel pairs of these particles, or opposite the 

 centre of the quadrilateral figure which they form*. 



Professor Dana has shown that the various fundamental forms 

 of crystals may be obtained by regarding the crystal as a mass 

 of bipolar molecules, of a spherical or spheroidal form, in con- 

 tact along certain lines, which are the conjugate axes or conju- 

 gate diameters of the spheroids. This conception of the consti- 

 tution of a crystal is, in a geometrical point of view, equivalent 

 to that which has now been given. For we have only to con- 

 ceive spheroids to be inscribed in the polyhedral figures of the 

 compound crystal molecules just supposed, to obtain the repre- 

 sentative spheroidal molecules of Prof. Dana; which will also 

 touch each other along similar conjugate diameters of the differ- 

 ent spheroids. It is not difficult to make out the various posi- 

 tions of equilibrium of the particles that must obtain in the 

 different fundamental forms, and the various physical conditions 

 of the particles upon which these forms must dependf. 



In his paper " On Cohesive Attraction," Professor Dana has 

 apparently put the explanation of the cleavage of crystals on the 

 true basis, by attributing it to alternations in the intensity of the 

 attraction in a series of parallel planes. If such alternations 

 really exist, we naturally seek for the explanation of them in 

 alternations of the mechanical condition of the molecules upon 



* This is only a partial view of the matter. 



t The hypothesis of a permanent polarity of atoms or molecules has 

 subserved a valuable purpose in linking together phenomena under one 

 physical conception in several departments of physical science ; but the 

 progress of science has materially tended of late to shake the confidence 

 reposed in it as a supposed truth of Nature. It will be conceded that it is 

 the dictate of true philosophy to hold it in abeyance until it shall have 

 become abundantly evident that the phenomena in question cannot be 

 deduced from the fundamental conception of the constitution of a mole- 

 cule, and of the primary forces of attraction and repulsion, to which all 

 other molecular phenomena can be referred. 



