368 /. D. Dana on Cohesive Attraction. 



form ; and if the prism has oblique angles instead of being rect- 

 angular, these lines of strongest attraction must have a corres- 

 ponding obliquity. Hence the angles referred 



/*v 



ing cohesive attraction, are angles between certain imaginary 

 lines, or axes, in whose direction the attraction is strongest. 



Again : the crystalline forms in nature are well known to have 

 fundamentally fixed relative dimensions, indicated by the modi- 

 fications they undergo (producing secondary forms) though not 

 necessarily apparent in the actual proportions of the crystal. 

 Thus a cube, which is equal in its dimensions, shows it in all its 

 modifications ; and in a prism, the inequality in the dimensions 

 is as exactly and precisely indicated by its modifications * The 

 relative dimensions belonging to the fundamental form of a sub- 

 stance, are often therefore easily calculated ; and the whole sci- 

 ence of crystals is thus based on rigorous mathematical laws. 



From these facts we may conclude therefore with respect to 

 solids, that Cohesive Attraction is characterized by fixed angles, 

 as regards the direction of its action, and by specific relations of 

 force in certain axial directions ; and it differs in these par- 

 ticulars for different substances. ,f 



These facts are the only hints which nature gives us respecting 

 the axial dimensions of molecules. We proceed on the only possi- 

 ble grounds for any conclusion on this point, when we infer that 

 molecules have corresponding relative dimensions with the crys- 

 talline forms, and the same specific angles between the fxnxda- 



A square prism and a cube as presented in nature, mav have actually the 

 Ume < hrm.nsH.ns, owing to the distortion of the one or the other. But the funda- 

 mental nature of their forms, may, notwithstanding, be obvious to the eye. In 

 the cube (having equal faces and axes) all the edges will have similar secondary 

 planes (that » these planes will be identical in their inclinations to the faces of 

 the cube) : in the prism, the secondary planes of the lateral edges will differ in their 

 indmat.ons from those of the terminal. In the cube, there may be a plane on 

 any edge equally inclined to the faces of the cube. In the prism, the plane on a 

 terminal edge will always he unequally inclined to the including faces; such* 



plane removes a part of the two including faces and these parts will in all cases 

 h a „„ » ,t.fi„.. ... _ . .... 5 r xhese explana- 



to those unac- 



crystais. 'iVoatisoo nn f>rvstnia snould be studie 

 «r a full exposition of this subject. 



1 Apparent exceptions to this principle are supposed to exist in the case of gla*, 

 tabasheer and some other substances that do not polarize light. They are. how- 



" 7" I"' ,al '1 u .' y so "regularly aggregated as to show no poi*..*— - - 

 and it their form is cqmnxal, no rings of polarization are to be expected. 



We have no facts to determine the form of the molecules in glass. As gw- » 

 (common as well as volcanic,) when slowly cooled, produces a different mater" , 



niM ement of the particles, eve 



strong. 



