58 CRYSTALLINE AGGREGATION. 



under the same circumstances, take a position nearly at 

 right angles to these curves. Certain groups of crystals 

 have been found in nature, which appear to show, by 

 their positions, that terrestrial magnetism has been active 

 in producing the phenomena they exhibit; indeed, 

 nearly all our mineral formations indicate the influences 

 of this, or some similarly acting power.* 



During rapid crystallisation, some salts as the sul- 

 phate of soda, boracic acicl, and arsenious acid crystallis- 

 ing in muriatic acid exhibit decided indications of 

 electrical excitement ; light is given out in flashes. We 

 have evidence that crystals exhibit a tendency to move 

 towards the light, and that crystallisation takes place 

 more readily, and progresses tfith greater activity in the 

 sunshine than in the shade. Professor Pliicker has 

 recently ascertained that certain crystals in particular 

 the cyanite " point very well to the north, by the mag- 

 netic power of the earth only. It is a true compass 

 ueedle ; and, more than that, you may obtain its decli- 

 nation." We must remember that this crystal, the 

 cyanite, is a compound of silica and alumina only. This 

 is the amount of experimental evidence which science has 

 afforded in explanation of the conditions under which 

 nature pursues her wondrous work of crystal formation. 

 We see just sufficient of the operation to be convinced 

 that the luminous star which shines in the brightness of 



# In the Memoirs of the Geological Survey of the United King- 

 dom, and of the Museum of Economic Geology, vol. i. 184(5, will bo 

 found a paper, by the author of this volume, On the Influences of 

 Magnetism on Crystallisation, and other Conditions of Matter, in 

 which the subject is examined with much care. See also Magnet 

 isme polaire d'une montagne de Chlorite schisteuse et de Serpentine : 

 Annales de Ghimie, vol. xxv. p. 327 ; Influence, du Magnetisme sur 

 {es actions chimiques, by 1' Abbe llendus; and also a notice of the 

 experiments of Hitter and Hansteen, " Analysees par M. (Ersted;" 

 also Effets du Magnetisme terrestre sur la precipitation de 1 J Argent, 

 observes par M. Muschman: Annales de Chimie, vol. xxxviii. p. 

 196201. 



