THE REJUVENESCENCE OF CRYSTALS. 287 



mind if we would explain tlie phenomena exhibited by this interest- 

 ing rock : 



A crystal may undergo the most profound internal changes, and 

 these may lead to great modifications of the optical and otlicr i>hys- 

 ical i)rop(ntics of the mineral ; yet, so long as a small — often a very 

 small — proportion of its molecules remain intact, the crystal may re- 

 tain, not only its outward form, but its capacity for growing and re- 

 pairing injuries. 



Crystals, like ourselves, grow old. Not only do they suffer from ex- 

 ternal injuries, mechanical fractures, and chemical corrosion, but from 

 actions wliich affect the whole of their internal structure. Under the 

 influence of the great inessures in the earth's crust, the minerals of 

 deep-seated rocks are completely permeated by fluids whicli cliemically 

 react upon them. In this way, negative crystals are formed in their 

 substance (similar to the beautiful '' ice-flowers" which are formed when 

 a block of ice is traversed by a beam from the sun or an electric lamj)), 

 and these become filled with secondary products. As the result of this 

 action, minerals, once perfectly clear and translucent, have acquired 

 cloudy, opalescent, iridescent, avanturine, and "schiller" characters; 

 and minerals, thus modified, abounded in the rocks that have at any 

 period of tlieir history been deep-seated. As the destruction of their 

 internal structure goes on, the crystals gradually lose more and more 

 of their distinctive optical and their physical properties, retaining how- 

 ever their external form, till at last, when the last of the original 

 molecules is transformed or replaced by others, they pass into those 

 mineral corpses known to us as "pseudoniorphs." 



But while crystals resemble ourselves in "growing old," and, at last, 

 undergoing dissolution, tliey exhibit the remarkable power of growing- 

 young again, which we, alas! never do. This is in consequence of the 

 following remarkable attribute of crystalline structures. 



It does not matter how far internal change and disintegration may 

 have gone on in a crystal; if only a certain small ])roportion of the 

 unaltered molecules lemain, the crystal may renew its ycmth and re- 

 sume its growth. 



When old and much altered crystals begin to grow again, the newly 

 formed material exhibits none of those marks of "senility'' to whicli I 

 have refeired. The sand grains that have been battered and worn into 

 microscopic pebbles and have been rendered cloudy by the development 

 of millions of secondary fluid cavities may have clear and fresh (piartz 

 deposited upon them to form crystals with exquisitely perfect faces 

 and angles. The white, (jlouded, and altered felds]»ar crystals nuiy be 

 enveloped by a zone of clear and transparent material, which has been 

 added millions of years after the first formation and the snbseiiuent 

 alteration of the original crystal. 



We are now in jiosition to explain the ])articnlar case which I have 

 thought of sufficient interest to claim your attention to-night. 



