1891.] on the Rejuvenescence of Crystals. - 253 



illustrated by many artificial crystals, and by the tourmalines, the 

 epidotes, and the felspars among minerals. The separate 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 

 crystallise in the same system. Thus crystals of the monoclinic 

 angite may be found surrounded by a zone of the rhombic enstatite ; 

 and crystals of a triclinic felspar may be found enlarged by an 

 outer shell of monoclinic felspar. 



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

 crystalline form, and an approximation in the dominant angles 

 (plesiomorphism), we may have zoning and intergrowth in the 

 crystals of substances which possess no chemical analogy whatever. 

 Thus, as Senarmont showed in 1856, a cleavage-rhomb of the natural 

 calcic carbonate (calcite), when placed in a solution of the sodic 

 nitrate, becomes enveloped in a zone of this latter substance ; and 

 Tschermak Las proved that the compound crystal thus formed behaves 

 like a homogeneous one, if tested by its cleavage, by its suscep- 

 tibility to twin lamellation, or by the figures produced by etching. 

 In the same way, zircons, which are composed of the two oxides of 

 silicon and zirconium, are found grown in composite crystals with 

 xenotime, a phosphate of the metals of the cerium and yttrium groups. 



These, and many other similar facts which 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 ; it may be 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 he hr&hen or mutilated in any way whatever, it possesses 

 the power of repairing its injuries during subsequent growth. 



As long ago as 1836, Frankenheim showed that if a drop of a 

 saturated solution be allowed to evaporate on the stage of a micro- 

 scope, the following interesting observations may be made upon the 

 growing crystals. When they are broken up by a rod, each fragment 

 tends to re-form as a perfect crystal ; and if the crystals be caused to 

 be partially redissolved by the addition of a minute drop of the 

 mother-liquor, further evaporation causes them to resume their 

 original development.* 



In 1842 Hermann Jordan showed that crystals taken fVom a 

 solution and mutilated, gradually became repaired or healed when 

 replaced in the solution.j" Jordan's observations, which were 



* Pogg. Ann., Bd. xxxvii. (1836). 

 t Mliller, Archiv for 1842, pp. 46-56. 



Vol. XIII. (No. 85.) s 



