284 THE REJUVENESCENCE OF CRYSTALS. 



that which includes the forms where the successive zoues are of differ- 

 ent, though analogous, chemical composition. In the case of the alums 

 and garnets, we may have various isomorphous compounds forming 

 the successive zones in the same crystal; while, in substances crystal- 

 lizing in other systems than the cubic, we hnd plesiomorphous com- 

 pounds forming the different inclosing shells. 



Such cases are illustrated by many artificial crystals aud by the 

 tourmalines, the epidotes, and the feldspars among minerals. The zones, 

 consisting of different materials, are sometimes separated by well 

 marked planes, but in other cases they shade imperceptibly into one 

 another. 



In connection with this subject it may be well to point out that 

 zoned crystals may be formed of two substances which do not crystal- 

 lize in the same system. Tlius, crystals of the monoclinic augite may 

 be found surrounded by a zone of the rhombic enstatite and crystals 

 of a triclinic felspar may be fouiul enlarged by a monoclinic feldspar. 



Still more curious is the fact that, where there is a similarity in crys- 

 talline form and an approximation in the dominant angles (plesiomor- 

 phisni), we may have zoning and intergrowth in the crystals of sub- 

 stances which possess no chemical analogy whatever. Thus, as Senar- 

 mont showed in 1856, a cleavage-rhomb of the uatural calcic carbonate 

 (calcite), when placed in a solution of the sodic nitrate, becomes en- 

 veloped in a zone of this latter substance, and Tschermak has proved 

 that the compound crystal thus formed behaves like a homogeneous 

 one, if tested by its cleavage, by its susceptibility to twin lamellation, 

 or by the figures produced by etching. In the same way, zircons, which 

 are composed of the two oxides of silicon aiul zirconium, are found 

 grown in composite crystals with xenotime, a phosphate of the metals 

 of the cerium and yttrium groups. 



These facts, and many similar ones wliich might be adduced, point 

 to the conclusion that the beautiful theory of isomorphism, as originally 

 propounded by Mitscherlich, stands in need of much revision as to many 

 important details, if not indeed of complete reconstruction, in the light 

 of modern observation and experiment. 



The second property of crystals to which I must direct your attention 

 is the following: 



If a crystal be broken or nuitilated in any way whatever, it possesses 

 the power of repairing its injnries during subsequent growth. 



As long ago as 1830, Frankenheim showed that, if a drop of a sat- 

 urated solution be allowed to evaporate on the stage of a microscope, 

 the following interesting observations may be made upon the growing 

 crystals. When they are broken up by a rod, eacli fragment tends to re- 

 form as a perfect crystal; and if the crystals be cansed to be partially 

 re-dissolved by the addition of a minute drop of the mother liquor, 

 further exaporation causes them to resume their original development 

 {Pogg. Ann., 1830, Bd. xxxvii). 



