84 THE BEGINNINGS OF LIFE. 



become rounded off and dissolve, because they are 

 more easily soluble than the others, while the arrago- 

 nite-shaped prisms go on increasing in size. When 

 the two kinds of crystals come into immediate contact, 

 the rhombohedral ones instantly become turbid, acquire 

 an uneven surface, and after a short time throw out 

 prisms from all parts of their surfaces. Contact with 

 foreign bodies also brings about the transformation of 

 the rhombohedrons while they are wet. If the drops 

 are so shallow that the liquid dries round the rhombo- 

 hedrons before they are disturbed, they will remain for 

 weeks without disintegrating, and bear gentle pressure 

 with foreign bodies without alteration; but stronger 

 pressure, or scratching, or the mere contact of a pris- 

 matic crystal of saltpetre, causes them to change, a 

 delicate film proceeding, as it were, from the point of 

 contact, and spreading itself over their surfaces ; they 

 then behave towards foreign bodies like a heap of fine 

 dust, but retain their transparency. The rhombo- 

 hedrons are also transformed, without alteration of 

 external appearance, when heated considerably above 

 iooC. : they then become much harder, because the 

 fine powder first produced bakes together into prismatic 

 crystals.' (loc. cit. p. 333.) 



These facts, together with those already cited, seem 

 to show clearly enough, not only that the crystalline 

 form of any crystallizable material is variable to a 

 remarkable extent when it is allowed to crystallize 

 under different conditions, but that, even when formed, 



