THE BEGINNINGS OF LIFE. 83 



which crystals of a different kind are produced, often lose 

 their transparency, and without alteration of external 

 form^ become changed into an aggregate of small crystals 

 of the latter kind: examples of this alteration of struc- 

 ture are afforded by sulphur, carbonate of calcium, 

 mercuric iodide, and many other bodies.' Again: 

 c Mercuric iodide separates from solution, and likewise 

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

 ing to the dimetric system; but when sublimed at a 

 higher temperature in sulphur-yellow, rhombic tables 

 of the monoclinic system. The red crystals turn yellow 

 when heated, and resume their red tint on cooling. 

 The yellow crystals obtained by sublimation retain 

 their colour when cooled ; but, on the slightest rubbing 

 or stirring with a pointed instrument, the part which 

 is touched turns scarlet, and this change of colour 

 extends with a slight motion, as if the mass were 

 alive, throughout the whole group of crystals as far 

 as they adhere together. 3 (Vol. ii. p. 332.) Then 

 again : Nitrate of potassium usually crystallizes in the 

 form of arragonite: but if a drop of the aqueous 

 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 

 of obtuse rhombohedrons of the calcspar form are 

 produced, just like those in which nitrate of sodium 

 crystallizes. As the two kinds of crystals increase in 

 size and approach one another, the rhombohedrons. 





