184 CAUSE OF ANGULAR SHAPES. 



exact are the polygonal shapes of crystals, that they 

 appear to be works of art, rather than natural for- 

 mations. Feeble as may appear the movements 

 of molecules visible under the microscope, yet the 

 power that moves them is more forcible than that 

 of gunpowder ; for the strongest cannon are burst 

 by the expansion of particles of water, on reducing 

 the temperature a few degrees below the freezing 

 point. Thus the terrestrial currents that produce 

 a change of particles of water into ice-crystals 

 exceed the force developed by the vibrations 

 excited by 1000 of heat. 



The angular and symmetrical forms of crystals 

 appear to result from the peculiar shape of each 

 kind of molecule, these molecules fitting to each 

 other electro-magnetically according to the angles 

 of their various sides. A cube is formed by 

 molecules arranged with rectangular sides, as is 

 manifest from the cleavages. The various angles 

 at which they unite may be indicated by sprink- 

 ling on a loadstone small nails with angular heads. 

 The sloping sides of the heads are drawn to the 

 sides of the loadstone, giving corresponding direc- 

 tion to the nails, which bristle out in various di- 

 rections, as represented in Fig. 63. l 



The polarization of watery sap in the vesicles of 



1 A speculative philosopher, the Rev. J. G. McVicar, LL.D., has 

 fancifully depicted various forms of molecules, adapting them to fit 

 together in different combinations, some of these curious forms resem- 

 bling the hour-glass-shaped waists of the belles of ancient times. 



