370 /. D. Dana on Cohesive Attraction. 



After these observations, I continue with the statement of facts 

 and the inferences they sustain. 



2. In crystals which have unequal axes, the physical qualities 

 of the crystal (such as color, hardness, lustre, &c.) are different 

 in unlike directions ; and they are uniformly alike in the direction 

 of equal axes. This symmetrical character indicates that 



or homologous axes unite. 



>f 



3. The electrical polarity of many crystals ; the occasional dis- 

 similarity of form in the opposite extremities of the same prism ; 

 the facts with regard to compound crystals, as well as direct ex- 

 periments with magnets on the process of crystallization, show 

 that 



III. The axes of cohesive attraction in molecules have oppo- 

 site polarity at opposite extremities ; — that is, the opposite poles 

 are positive and negative, or north and south, as these terms are 

 ordinarily used.* 



4. Many geniculated crystals, are geniculated alike at equal dis- 

 tances from the middle of the prism, (or, are like a column bent 

 alike at two places equidistant from the middle.) They have be- 



character of the polarization in crystals. The effect of this flattening of the sur- 

 face of the molecule by pressure is probably seen in the curious crystals of ana- 

 cime,« whose structure has been developed by this distinguished philosopher. l 

 the case of glass and other substances, that receive the power of polarization i) 



Pressure, it would seem from the facts just stated, that it depends upon a c hang 

 y compression of the spherical molecule to a spheroidal or compressed shape 

 the shape is a result of pressure ; but as in crystals, the polarization may be 

 pendent on the shape. 



The polarization of molecules will depend not only on their form, tart« '•?° 

 their nature, whether uniform in character throughout, or differing in density in 

 the centre to the circumference and with a definite inverse relation to the ,en S ' 

 of unequal axes. As the latter supposition is altogether the most probable, t 

 sidering that molecules are centres of attraction, the form of the m°lecuie 

 the form deduced from polarization should not be the same until this law or 

 tion is understood and duly appreciated. 



* By polarity we imply simply that relation between the diametrically opp ^ 

 parts, A and B, of a molecule, by which the part A of one molecule unites 

 other molecule by the part B, and repels the part A. This property APF ar * in pa- 

 one of (he fundamental qualities of matter or force, as no exhibition of lore 

 ture is divested of it. In solidification this polarity is exerted most strong y 



fixed axes 



polarity 



«, do not polar i*j 



a Crystals of the tesseral system to which anaieime belongs, do no p ^ ^ 



light; but in specimens of this mineral, a peculiar system of nmjs ^ j 



detected by Brewster: they suggest at once, that like certain cooie a rfeJrt 

 glass, the structure has arisen from a change of form in the molecules a i 



on tension. 



