106 CRYSTALIZATION. SBCT. XIV. 



and takes a different form. Heat appears to have a 

 great influence on the phenomena of crystalization, not 

 only when the particles of matter are free, but even 

 when firmly united, for it dissolves their union and gives 

 them another determination. Professor Mitscherlich 

 found that prismatic crystals of sulphate of nickel (N. 161 ) 

 exposed to a summer's sun in a close vessel, had their 

 internal structure so completely altered without any ex- 

 terior change, that when broken open they were com- 

 posed 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 crystaline 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 crys- 

 tals totally different in form from the whole crystals ; 

 and prismatic crystals of zinc (N. 162) 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 molecular attrac- 

 tion 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 temper- 

 ature at which these changes are effected, that there 

 is probably no portion of inorganic matter that is not in 

 a state of relative motion. 



Professor Mitscherlich's discoveries with regard to 

 the forms of crystalized substances, as connected with 

 their chemical charcter, have thrown additional light on 

 the constitution of material bodies. There is a certain 

 set of crystaline forms which are not susceptible of 



