1891.] on the Bejuvenescence of Crystals. 251 



a crystalline mass ; and these phenomena, which have been revealed to 

 the student of natural crystals, are of such a kind that we can scarcely 

 hope to reproduce them in our test-tubes and crucibles. 



But if we cannot expect to imitate all the effects which have in 

 this case been slowly wrought out in Nature's laboratory, we can at 

 least investigate and analyse them; and, in this way, it may be 

 possible to show that phenomena like those in question must result 

 from the possession by crystals of certain definite properties. Each 

 of these properties, we shall see, may be severally illustrated and 

 experimentally investigated, not only in natural products, but in the 

 artificially-formed crystals of our laboratories. 



In order to lead up to the explanation of the curious phenomena 

 exhibited by the rock mass in question, the first property of crystals 

 to which I have to refer may be enunciated as follows : — 



Crystals possess the power of resuming their growth after interruption ; 

 and there appears to he no limit to the time after which this resumption 

 of growth may take place. 



It is a familiar observation that if a crystal be taken from a 

 solution and put aside, it will, if restored after a longer or shorter 

 interval to the same or a similar solution, continue to increase as before. 

 But geology affords innumerable instances in which this renewal of 

 growth in crystals has taken place after millions of years must have 

 elapsed. Still more curious is the fact, of which abundant proof can 

 be given, that a crystal formed by one method may, after a prolonged 

 interval, continue its growth under totally diftbrent conditions and . 

 by a very different method. Thus crystals of quartz, which have 

 clearly been formed in a molten magma, and contain enclosures of 

 glass, may continue their growth when brought in contact with solu- 

 tions of silica at ordinary temperatures. In the same way, crystals 

 of felspar which have been formed in a mass of incandescent lava, 

 may increase in size when solvent agents bring to them the necessary 

 materials from an enveloping mass of glass, even after the whole mass 

 has become cold and solid. 



It is this power of resuming growth after interruption, which leads 

 to the formation of zoned crystals, like the fine specimen of amethyst 

 enclosed in colourless quartz, which was presented to the Royal 

 Institution seventy years ago by Mr. Snodgrass. 



The growth of crystals, like that of plants and animals, is 

 determined by their environment ; the chief conditions affecting their 

 development being temperature, rate of growth, the supply of 

 materials (which may vary in quality as well as quantity), and the 

 presence of certain foreign bodies. 



It is a very curious circumstance that the form assumed by a 

 crystal may be completely altered by the presence of infinitesimal 

 traces of certain foreign substances — foreign substances, be it remarked, 

 which do not enter in any way into the composition of the crystallising 

 mass. Thus there are certain crystals which can only be formed in 

 the presence of water, fluorides, or other salts. Such foreign bodies, 



