108 CRYSTALLIZATION. SECT. XIV. 



form. Crystals of sulphate of magnesia and of sulphate of zinc, 

 gradually heated in alcohol till it boils, lose their transparency 

 by degrees, and when opened are found to consist of innumerable 

 minute crystals totally different in form from the whole crystals ; 

 and prismatic cr} 7 stals of zinc (N. 165) are changed in a few 

 seconds into octahedrons by the heat of the sun : other instances 

 might be given of the influence of even moderate degrees of tem- 

 perature on molecular attraction in the interior of substances. 

 It must be observed that these experiments give entirely new 

 views with regard to the constitution of solid bodies. We are 

 led from the mobility of fluids to expect great changes in the 

 relative positions of their molecules, which must be in perpetual 

 motion even in the stillest water or calmest air ; but we were not 

 prepared to find motion to such an extent in the interior of 

 solids. That their particles are brought nearer by cold and 

 pressure, or removed farther from one another by heat, might be 

 expected; but it could not have been anticipated that their 

 relative positions could be so entirely changed as to alter their 

 mode of aggregation. It follows, from the low temperature at 

 which these changes are effected, that there is probably no por- 

 tion of inorganic matter that is not in a state of relative motion. 

 Professor Mitscherlich's discoveries with regard to the forms of 

 crystallised substances, as connected with their chemical cha- 

 racter, have thrown additional light on the constitution of 

 material bodies. There is a certain set of crystalline forms which 

 are not susceptible of variation, as the die or cube (N. 166), 

 which may be small or large, but is invariably a solid bounded 

 by six square surfaces or planes. Such also is the tetrahedron 

 (N. 167) or four-sided solid contained by four equal-sided 

 triangles. Several other solids belong to this class, which is 

 called the Tessular system of crystallisation. There are other 

 crystals which, though bounded by the same number of sides, 

 and having the same form, are yet susceptible of variation ; for 

 instance, the eight-sided figure with a square base, called an 

 octahedron (N. 168), which is sometimes flat and low, and some- 

 times acute and high. It was formerly believed that identity of 

 form in all crystals not belonging to the Tessular system indi- 

 cated identity of chemical composition. Professor Mitscherlich, 

 however, has shown that substances differing to a certain degree 

 iii chemical composition have the property of assuming the same 



