SECT, xiv.] CRYSTALLISATION. 125 



they were composed internally of octahedrons with square 

 bases. The original aggregation of the internal particles had 

 been dissolved, and a disposition given to arrange themselves 

 in a crystalline 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 differ- 

 ent in form from the whole crystals ; and prismatic crystals of 

 zinc (N.I 62) 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 temperature on mole- 

 cular 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 tempera- 

 ture at which these changes are effected, that there is pro- 

 bably no portion 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 character, have thrown additional light on the con- 

 stitution of material bodies. There is a certain set of crys- 

 talline forms which are not susceptible of variation, as the 

 die or cube (JST. 163), which may be small or large, but is in- 

 variably a solid bounded by six square surfaces or planes. 

 Such also is the tetrahedron (N. 164) or four-sided solid 

 contained by four equal-sided triangles. Several other solids 

 belong to this class, which is called theTessular system of cry s- 



