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op 



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THE BEGINNINGS OF LIFE. 



83 



ucnce of 



-ture of 

 ? cited 



vari{ 



which crystals of a difFerent kind are produced^ often lose 



afnii. their transparency, and ^without alteration of external 





form^ become changed Into an aggregate of small crystals 



c^learly tb ^ of the latter kind : examples of this alteration of struc- 



in §ome 



cast! 



1 



n Jeterinii; 

 compared 



ture are afforded by sulphur, carbonate of calciurrij 

 mercuric iodide, and many, other bodies.' Again: 

 ^Mercuric Iodide separates from solution, and likewise 



V. 



.^ 



^e chemist 



sublimes at a very gentle heat, in scarlet tables belong- 

 ing to the dimetric system , but when sublimed at a 

 ^i^y.pi^s: higher temperature in sulphur-yellow, rhombic tables 



^ncies of the 

 the 



\ 



ii 



of the monoclinic system. The red crystals turn yellow 



:i pie, by show: when heated, and resume their red tint on cooling, 

 already in ei The yellow crystals obtained by sublimation retain 



ghout 

 the 



as they adhere together/ 



ryofCheir^ 



3f causes wl their colour when cooled ; but, on the slightest rubbing 



t 



ition of temp or stirring with a pointed instrument, the part which 



h in some(^ ^s touched turns scarlet, and this change of colour 



1 J u gpj-ea^t extends with a slight motion, as if the mass were 



alive, throughout the whole group of crystals as far 



(Vol. ii. p. '^'>^%>) Then 

 again : — ^ Nitrate of potassium usually crystallizes in the 

 form of arragonite : but if a drop of the aqueous 



r 



solution of this salt be left to evaporate on a glass 

 plate and the crystallization observed under the micro- 

 scope, it will be found that, side by side with the 

 prismatic crystals at the edge of the drop, a number 

 p,,.. of obtuse rhombohedrons of the calcspar form are 

 '^^ ^^ al ^^^^ produced, just like those in which nitrate of sodium 

 beautif^^^ ^^^^ crystallizes. As the two kinds of crystals increase in 

 Ltions of ^^^1^0 size and approach one another, the rhombohedrons 



■) 



we 



find 



cd at one 

 that ternF^' 



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ir g' 



