COHESIVE ATTRACTION. 627 



II. COHESIVE AND CAPILLARY ATTRACTION; GRAV- 

 ITATION. 



1. Crystallization. — The power of cohesion acting in solidifica- 

 tion, and that in crystallization, appear to be identical. Snow, ice, bar- 

 iron, trap, granyte and even solid spermaceti are crystallized in their 

 intimate structure. Iron and granyte show it in the angular grains 

 which make up the mass, and which may be observed on a surface of 

 fracture; and ice, in the frosty covering of windows, and the prisms 

 which shoot across a surface on freezing, as well as in the vertical 

 columns into which it sometimes breaks when the ice of a pond melts 

 in spring. Quartz exhibits it in its prismatic and pyramidal crystals 

 (p. 55). The fact can thus be proved for all mineral solids, except it 

 be those of a glassy nature ; and even these are probably no excep- 

 tion to the principle that solidification is crystallization. 



Crystallization is exhibited (1) in the angular solids it produces, 

 called crystals, and (2) in the tendency to cleave or divide in one or 

 more directions, called cleavage. 



Crystals. — Some of the forms of crystals are illustrated on the 

 early pages of this work (pp. 53-59). Crystals are formed when 

 substances cool from fusion (as when melted sulphur cools) ; or 

 solidify from solution (as in the evaporation of a solution of alum) ; 

 or become condensed from the state of vapor (as in the formation of 

 snow from vapor of water). But it is usually requisite for perfection 

 that the process should go forward with extreme slowness, free from 

 all disturbing" causes, and with space for the crystals to expand. Cav- 

 ities in rocks are often lined with crystals, while the rock itself is but 

 a compact mass of crystalline grains. 



Long-continued heat, short of fusion, favoring a slow aggregation 

 of the particles, sometimes produces crystals, or a crystalline structure. 

 Heating steel to a certain temperature, short of that required for 

 fusion, changes the fineness of the grains, — which is a change of crys- 

 talline texture. 



Cleavage. — Cleavage is usually parallel to one or more planes or 

 diagonals of the fundamental form. 



The minerals mica and gypsum are examples of very easy cleavage. 

 Calcite has easy cleavage in three directions, making a fixed angle 

 (105° 5') with one another, parallel to the faces of the fundamental 

 rhombohedron. Feldspar has easy cleavage in one direction, and in 

 another a second cleavage, a little less perfect, at right angles, or 

 nearly so, with the first. Quartz has no distinct cleavage. 



Cleavage in rocks. — Rocks may derive a cleavage-structure from 



