/ 



378 /. D. Dana on Cohesive Attraction 



cahedron; that of Boonville, New York, occurs in short six- 

 sided prisms. That of the Rossie lead region in complex com- 

 binations of different secondary planes with the primary. 



XIV. These facts indicate, that the variations of attraction, 

 producing secondary forms, depend often on surrounding bodies 

 favoring the concentration or diffusion of the attracting J one; 

 and the causes often act simultaneously in nature over wide area*. 



15. It is also a usual truth, that crystals are sometimes very- 

 much lengthened beyond their normal proportions, and in other 

 cases, very much shortened. When attached, they are length- 

 ened or shortened in the direction of the axis of attachment; 

 and if obliquely attached, they are distorted in this direction. 

 These facts, which are of common observation, show that 



XV. In an enlarging crystal, one axis (or two) may have the 

 action of attraction more accelerated or retarded than another by 

 extrinsic influence, and this acceleration or retardation affects 

 equally all crystals forming together under common circum- 

 stances. 



16. The peculiarities of cleavage give us information on an- 

 other point respecting cohesive attraction. The facility of cleav- 

 age in prisms differs in the direction of unlike axes. Topaz 

 cleaves easily parallel to the base of the prism, and not at all m 

 other directions. This difference does not depend upon the rela- 

 tive strength of attraction in the unequal axes ; for it is often the 

 reverse of this. Again, while some of the hardest substances 

 have perfect cleavage, other soft species have none. 



If then this quality has no relation to the strength of the at- 

 traction which unites molecule to molecule, it must depend on 

 some peculiarity in the manner in which this force acts. This 

 force may act in two ways :— either continuously, or intermit- 

 ted ly ; and the latter mode only, could produce the result id 

 view. The action of force in nature appears to be generally in- 

 termittent. Alternate action and comparative inaction, with cor- 

 responding results, are every where exemplified in organic BJjJr' 

 and it is therefore no anomaly that 'it should be exemplified m 

 the inorganic kingdom.* 



flow 



* The successive layers in wood, the periodical reproduction of leaves or ^ 



ers and of young in animals, and the seriate arrangement of parts i"™ 8 "^ 

 and animals all illustrate intermittent growth, tn some zoophytes the Duo* 

 in successive series of two. fimr „ r =1- ,.- =^^ Q ^ti.„r fi**>d number; "i 



irni 

 th«f 



successive series of two, four or six, or some other fixed number, in 

 ses opposite sides alternate in budding, or when there are several row*, 

 uud m succession ; and these are examples of intermittent action. n w r ved. 



I he spiral arrangement of leaves in veg.-tation, as [ have elsewhere «• ^ 

 l*uopli.. p . 89,) is another illustration of intermittent growth ; for here i ■«. ^ 

 ent sides of the growing plant (five, in many plants, and six in many otner , 



