82 THE BEGINNINGS OF II FE. 



it would be aflForded by the facts which we are now 

 about to cite, referring to the influence of variation in 

 ' conditions,' even upon the structure of a fully formed 

 crystal. Such facts, however, are cited principally for 

 the purpose of showing, more clearly than we have 

 hitherto done^, how very potent in some cases is the 

 influence of varying ^conditions' in determining the 

 nature of crystalline forms, as compared with that 

 assignable to the inherent tendencies of the matter 

 itself. The citation of a few of the many phenomena 

 which are familiar enough to the chemist will serve 

 to make plain this general principle, by showing that 

 the whole nature of a crystal already in existence 

 may be changed by the action of causes which seem 

 the most trivial: a slight elevation of temperature, 

 or even the most delicate touch, in some cases, is 

 capable of initiating changes which spread through 

 their entire substance, or throughout a whole aggre- 

 gate of cohering crystals 2. In the same article on 

 ^ Dimorphism^ in Watts's 'Dictionary of Chemistry,' to 

 which we have already referred, we find the follow- 

 ing statement : — ' Crystals formed at one particular 

 temperature, and then exposed to that temperature at 



1 See p. 57. 



^ The forms displayed by cohering crystals are often most beautiful, 

 and sometimes strikingly resemble, in their general outlines, those of 

 shrubs or trees. We need only refer to the beautiful forms assumed 

 by snow crystals, to the tree-like ramifications of ice-crystals on the 

 window pane, or to the lead and silver ' trees ' which delight so many 

 in their childhood. 



