CRYSTALIJZATION. 



273 



iiif;^ it iu all. If we complete the pile, we shall get tlie triangular 

 pyramid represented by Fig. -4, where each of the three sides is a right- 

 angled triangle, while the base is an (equilateral triangle. It will be 

 seen that in the laces of this pyramid each ball (except those outside) 

 is touched by four others. In fact, the arrangement in these faces is 

 the same as in the base of the former pyramid; and the two arrange- 

 ments are really identical in the interior, only one has to be turned 

 over in order to bring it into parallelism with the other. Fig. '2 rep- 

 resents half a regular octahedron; Fig 4 the corner of a cube. Elli})- 

 soids if they are all equal and similiar to one another can be i)acked 

 in precisely the same way, so that each is touched by twelve others, 

 l)rovided their axes are kept j)arallel to each other — that is, if they 

 are all oriented alike. This, then, by the laws of energy, will be the 

 arrangement which the molecules will assume in consequence of mutual 

 attraction, in i»assing from a fluid to a solid state. 



^ext, let us see how the packing of the molecules will affect the ex- 

 ternal form. And here I bring in the surface tension. We are famil- 

 iar with the eflects of this force in the ease of liquids, and if we adopt 

 the usually received theory of it, we must have a surface tension at the 

 boundary of a solid, as well as at the surface of a li([uid, [ know of 

 no actual measures of the surface tension of solids; but (»),uincke has 

 given us the surface tensions of a number of substances at temiieratures 

 near tlicir points of soliditication, in dynes per lineal centimeter, as 

 follows: 



Platimiiii (il.S 



fi.)](l 983 



ZiiK- 800 



Till 587 



Mercury 577 



Lead 448 



Antimony 244 



Borax 2^2 



.Sodiiiiu carlionatc 20(! 



Sodium chloric Ic 114 



Water Sd. 2 



Selenium 70. I 



Silver 419 ; Sulphur 41.;} 



IJisnmth 382 1 rhosi)horns 41.1 



Potassium 364 Wax 33. 4 



Sodium 253 [ 



The surface tensions of most of the solids are probably greater than 

 these, for the surface tension generally diminishes witli increase of 

 temperature: and you see that they amount to very considerable Ibrces. 

 II. Mis. 114^ 18, 



