368 J. D. Dana on Cohesive Attraction. 



axes, as these are only imaginary lines of concentration of force; 

 the other parts of the molecule must necessarily have attracting 

 force though to a less amount than along the axial lines * 



The fact that crystals are formed by the superposition of mole- 

 cules by axial attraction, is a matter of observation. In an evap- 

 orating brine we may see the minute cube of salt enlarging with- 

 out change of form, a fact which implies that ranges of particles 

 are added regularly to each side. In a drop of sea water under 

 the microscope, we may watch the growing crystal of gypsum, 

 and see its rhombic and arrow-head forms as perfect in the small- 

 est visible point, as afterwards when more enlarged ; proving 

 again that the particles are added in fixed lines, since in no other 

 way could there be this constancy of angle. It is proved again 

 by finding many instances in calc spar, quartz and other minerals, 

 of crystals with internal layers of another mineral which were 

 deposited on the faces of the crystal during an intermission in 

 their progress; showing the form of the crystal in its earlier 

 stages. Hence we may not doubt the reality of the axial lines 

 of cohesive attraction. 



Brewster, in the course of his splendid researches on the opti- 

 cal phenomena of crystals, has shown that in some instances the 

 particles are in a state of tension, as by compression. In a re- 

 cent article on the topaz,t he describes the occurrence, in certain 

 crystals, of extremely minute cavities, which indicate by means 

 ot polarized light, that the parts adjoining have been acted upon 

 by a compressing force. Long since he observed respecting the 

 diamond that its crystals,— which are peculiar in having convex 

 faces,— exhibit, as he states, "an imperfect, doubly-refracting 

 structure, as if aggregated by irregular forces, and compressed or 



ible va- 

 in an 



The several axial conditions illustrated in crystals include all the possibl 

 nations of the three d.arrjeters of spheroids, as is mentioned by the author »- - 

 article m this Journal, vol. „ x , 1336, p. 282, and Mineralogy, 2nd edition, p. »• 

 J hey are as follows (us.ng the term axes for the diameters having rectangular 

 intersections; and diameters, for the diameters havin<- oblique intersections.) 



.} %t ere ~7 J-' 66 c ? njUgate a *es; equal, (I.) Cube. 



11. Ellipsoid of revolution. 



A. Three conjugate axes, the two lateral 



n e 3il' . • (2.) Bight square prism. 



B. Three equal conjugate diameters, [with 



tit „,,. e< l ua J obl 'q» e angles of intersection,] (3.) Rliombohedron. 

 111. Ellipsoid, not of revolution. 



A. Three conjugate axes, unequal, (4.) Right rectangular prfr"- 



U. A vertical axis, and two equal conju- 

 gate diameters, , 5 .\ jn^t rhombic prism. 



C. A vertical axis, and two unequal con- ° , ._ 

 jugate diameters, ' (6.) Riahtrhomboidal pr*™- 



D. Three conjugate diameters, two equal, (7.) Oblique rhombic prism- 

 tu. Ihree conjugate diameters, unequal, (8.) Oblique rhomboidai pr 



t L. E. and D. Phil. Mag., August, 1347, xxxi, 101. 



