56 SIZE OF CRYSTALS. 



prisms in arragonite. The molecular arrangements here 

 are not, however, of equal stability, and one form is 

 evidently forced upon the other, and is abandoned by it 

 on the slightest disturbance. When a prism of arra- 

 gonite is heated it breaks up into the rhombs of common 

 calc spar, at a temperature far below that at which the 

 carbonate of lime is decomposed ; but no alteration of 

 temperature can convert calc spar into arragonite. 



Crystals are found in the most microscopic character, 

 and of an exceedingly large size. A crystal of quartz at 

 Milan is three feet and a quarter long, and five feet and 

 a half in circumference, and its weight is 870 pounds, 

 Beryls have been found in New Hampshire measuring* 

 four feet in length.* 



In the dark recesses of the earth, where the influences 

 which produce organisation and life cease to act, a 

 creative spirit still pursues its never-ending task of giving 

 form to matter. 



The science of crystallogeny,f embracing the theoreti- 

 cal and practical question of the causes producing these 

 geometric forms, has in various *ways attempted to ex- 

 plain the laws according to which molecules arrange 

 themselves on molecules in perfect order, giving rise to 

 a rigidly correct system of architecture. But it cannot 

 be said that any theory yet propounded is sufficiently 

 exact to embrace the whole of the known phenomena, 

 and the questions, What is crystallogenic attraction, 

 and what is the physical nature of the ultimate particles 

 of matter, are still open for the inquiries of that 

 genius which delights in wrestling with the secrets of 

 nature. 



* A System of Mineralogy, comprising the most recent discoveries) 

 "by James D. Dana, A.M., New York, 1844. 



f Crystallogeny, or the formation of crystals, is the term em- 



ployed by Dana, in his admirable work quoted above: wliosr 

 remarks on Theoretical Crystallogeny, p. 71, are well worthy of till 



attention 



