CRYSTALLINE AND MOLECULAR FORCES. 263 



their lengths in a common direction, such elongated and flat particles 

 would, when solidified, certainly produce a cleavage. 



Plausible as this is, it is not the proper explanation, the cleavage 

 of the slate-rocks being demonstrably not crystalline, but, as shown 

 by Sharpe, Sorby, Haughton, and myself, due to pressure. 



The outward forms of these crystals are various and beautiful. A 

 quartz-crystal, for example, is a six-sided prism, capped at each end 

 by six-sided pyramids. Rock-salt, with which your neighbors in 

 Cheshire are so well acquainted, crystallizes in cubes ; and it can be 

 cloven into cubes until you cease to be able to cleave further for the 

 very smallness of the masses. Rock-salt is thus proved to have three 

 planes of cleavage at right angles to each other. Iceland spar has 

 also three planes of cleavage, but they are oblique instead of rectan- 

 gular, the crystal being, therefore, a rhomb instead of a cube. Various 

 crystals, moreover, cleave with different facilities in different direc- 

 tions. A plane of principal cleavage exists in these crystals, and is 

 accompanied by other planes, sometimes of equal, sometimes of un- 

 equal value as regards ease of cleavage. Heavy spar, for example, 

 cleaves into prisms, with a rhombus or diamond-shaped figure for a 

 base. It cleaves with greatest ease across the axis of the prism, the 

 other two cleavages having equal values in this respect. Selenite 

 cleaves with extreme facility in one direction, and with unequal facili- 

 ties in two other directions. 



Looking at these beautiful edifices and their internal structure, the 

 pondering mind has submitted to it the question, How have these crys- 

 tals been built up ? What is the origin of this crystalline architecture ? 

 Without crossing the boundary of experience, we can make no attempt 

 to answer this question. We have obtained clear conceptions of polar 

 force ; we know that polar force may be resident in the molecules or 

 smallest particles of matter we know that by the play of this force 

 structural arrangement is possible. What, in relation to our present 

 question, is the natural action of a mind furnished with this knowl- 

 edge ? Why, it is compelled by its bias toward unity of principle to 

 transcend experience, and endow the atoms and molecules of which 

 these crystals are built with definite poles, whence issue attractions 

 and repulsions for other poles. In virtue of this attraction and repul- 

 sion some poles are drawn together, some retreat from each other ; 

 atom is thus added to atom, and molecule to molecule, not boister- 

 ously or fortuitously, but silently and symmetrically, and in accord- 

 ance with laws more rigid than those which guide a human builder 

 when he places his bricks and stones together. From this play of in- 

 visible particles we see finally growing up before our eyes these ex- 

 quisite structures, to which we give the name of crystals. 



In the specimens hitherto placed before you the work of the atomic 

 architect has been completed ; but you shall see him at work. In the 

 first place, however, I will take one of his most familiar edifices, and 



