J. D. Dana on Cohesive Attraction. 377 



XIII. The direction of cleavage may indicate in any species of 

 matter which set of axes is dominant, the primary, or a sec- 

 tmdary set. 



In the preceding paragraphs, after ascertaining the general po- 

 lar action of cohesive attraetion, we have pointed out the modifi- 

 cations of condition this attraction undergoes, the simple ratios 

 presented by these modifications, and their dependence on the 

 formation of intermediate axes. Cohesive attraction instead of 

 being a constant force, as might be inferred from the ordinary 

 definitions, appears therefore to be complex in its actions, yet 

 simple in the general laws by which this complexity is produced. 



14. The absence or presence of secondary planes, and their 

 character, are known in some instances to depend on external 

 circumstances. Electric currents, the nature of the supporting 

 rock, or the condition of the solvent are determining causes. A 

 floating crystal has been seen to form secondary planes on be- 

 coming attached. Crystallizing in a thick pasty mass as Beudant 

 has observed, will generally afford the simpler forms. The most 

 complex are usually the purest crystals, such as have been pro- 

 duced in the most quiet circumstances. Implanted crystals are 

 sometimes rendered more complex during the last stages of their 

 increase than they were before. These facts show some of the 

 ways by which modifications in the condition of the attraction 

 «* the primary axes are produced. The presence of foreign ma- 



'arity in the primary axes, will occasion the formation of simple 

 forms. But if there is nothing to sustain or excite this concen- 

 tration, or the action is quiet, or if bodies around induce it, owing 

 to their own condition, the attraction becomes more diffused, and 

 secondary axes multiply * All the crystals of a locality or re- 

 gion have usually the same form. The constancy of certain 

 forms in some species is evidence of the pecu bar susceptibilities 

 °f the molecules of those substances. Thus the scale spar in the 

 limestone of Wknnrt. has the dog-tooth shape, the scalene dode- 



* The theory above offered with regard to '^ «n g m of econ ( l y 1 ne . 

 »*ar that presented in 1839 to the Royal Society, by Prof. Necker. Howfcrttjy 

 »« identical, I cannot decide with certainty from ** br J*J a «g 6 ° r &" $$£ 

 w »»chl have seen in the Philosophical Magaz.ne, ^ "I'i»| form, of tbell 

 Eposes that there is a tendency ,n crystals to take the 9h««Md form. of the., 

 •ofcnta. ; that when the secondary ax.-s are destroyed by *£*•"« * ', „o uZ 

 7.vs«al S are produced. According to the v.ew here 1™»«g**™£ ''"?„ " 

 fcnev of this kind recognized; the concentration of the prima) «<* JJ J J ?« 

 »"!ple forms is the ordinary condition of the molecule* of m ^ v < v 



•^fiction ; and by this concentration, however produced f.:'^™^*™ 

 " <eir force. The view 1 had gathered from the ■^{^^JSE New 

 ■"*', is given in a note to page 100 in the author a Mineral, , 2nd edit., iNew 



Haven 1U4 t 



Haven, lb44. 



Skcond Se RIE5 , Vol. IV, No. 12.— Nov., 1847. 4b 



