84 



NATURE 



[May 28, 189] 



quality as well as in 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 crystallizing mass. Thus 

 there are certain crystals which can only be formed in the 

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

 bodies, which exercise an influence on a crystallizing sub- 

 stance without entering into its composition, have been 

 called by the French geologists " mineralizers." Their 

 action seems to curiously resemble that of diastase, and 

 of the bodies known to chemists as " ferments," so many 

 of which are now proved to be of organic origin. 



Studied according to their mode of formation, zoned 

 crystals fall naturally into several different classes. 



In the first place, we have the cases in which the 

 successive shells or zones differ only in colour or some 

 other accidental character. Sometimes such differently 

 coloured shells of the crystal are sharply cut off from one 

 another, while in other instances they graduate imper- 

 ceptibly one into the other. 



A second class of zoned crystals includes those in 

 which we find clear evidence that there have been pauses, 

 or, at all events, changes in the rate of their growth. 

 The interruption in growth may be indicated in several 

 different ways. One of the commonest of these is the 

 formation of cavities filled with gaseous, liquid, or 

 vitreous material, according to the way the crystal has 

 been formed — by volatilization, by solution, or by fusion ; 

 the production of these cavities indicating rapid or ir- 

 regular growth. Not unfrequently it is clear that the 

 crystal, after growing to a certain size, has been corroded 

 or partially resorbed in the mass in which it is being 

 formed, before its increase was resumed. In other cases, 

 a pause in the growth of the crystal is indicated by the 

 formation of minute foreign crystals, or the deposition of 

 uncrystallized material along certain zonal planes in the 

 crystal. 



Some very interesting varieties of minerals, like the 

 Cotterite of Ireland, the red quartz of Cumberland, and 

 the spotted amethyst of Lake Superior, can be shown to 

 owe their peculiarities to thin bands of foreign matter 

 zonally included in them during their growth. 



A curious class of zoned crystals arises when there is a 

 change in the habit of a crystal during its growth. Thus, 

 as Lavalle showed in 1851 {Bull. Geol. Soc. Paris, 2me. 

 s^r., vol. viii. pp. 610-13), if an octahedron of alum be 

 allowed to grow to a certain size in a solution of that 

 substance, and then a quantity of alkaline carbonate be 

 added to the liquid, the octahedral crystal, without 

 change in the length of its axes, will be gradually trans- 

 formed into a cube. In the same way, a scalenohedron 

 of calcite may be found inclosed in a prismatic crystal of 

 the same mineral, the length of the vertical axis being 

 the same in both crystals. 



By far the most numerous and important class of zoned 

 crystals is that which includes the forms where the suc- 

 cessive zones are of different, though analogous, chemical 

 composition. In the case of the alums and garnets, we 

 may have various isomorphoiis compounds forming the 

 successive zones in the same crystal ; while, in substances 

 crystallizing in other systems than the cubic, we find 

 plesiomorphous compounds forming the different enclosing 

 shells. 



Such cases are illustrated by many artificial crystals, 

 and by the tourmalines, the epidotes, and the felspars 

 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 

 NO. I I 26, VOL. 44] 



which do not crystallize in the same system. Thus, crys- 

 tals of the monoclinic augite may be found surrounded 

 by a zone of the rhombic enstatite ; and crystals of a 

 triclinic felspar may be found enlarged by a monoclinic 

 felspar. 



Still more curious is the fact that, where there is a 

 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 solu- 

 tion of the sodic nitrate, becomes enveloped in a zone of 

 this latter substance ; and Tschermak has proved that 

 the compound crystal thus formed behaves like a homo- 

 geneous one, if tested by its cleavage, by its suscepti- 

 bility 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 facts, and many similar ones 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, if not, indeed, of complete reconstruc- 

 tion, 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 mutilated in any way what- 

 ever, 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 microscope, 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 re-dissolved by the addition of a 

 minute drop of the mother liquor, further evaporation 

 causes them to resume their original development {Pogg. 

 Ann., Bd. xxxvii., 1836). 



In 1842, Hermann Jordan showed that crystals taken 

 from a solution and mutilated gradually became repaired 

 or healed when replaced in the solution {Miiller Archiv. 

 filr 1842, pp. 46-56). Jordan's observations, which were 

 published in a medical journal, do not, however, seem to 

 have attracted much attention from the physicists and 

 chemists of the day. 



Lavalle, between the years 1850 and 1853,^ and Kopp, 

 in the year 1855, made a number of valuable observations 

 bearing on this interesting property of crystals {Liebig 

 Ann., xciv., 1855, pp. 118-25). In 1856 the subject was 

 more thoroughly studied by three investigators who pub- 

 lished their results almost simultaneously : these were 

 Marbach {Compt. rend., xliii., 1856, pp. 705-706, 800-802), 

 Pasteur {ibid., pp. 795-800), and Senarmont {ibid., p. 799). 

 They showed that crystals taken from a solution and 

 mutilated in various ways, upon being restored to the 

 liquid became completely repaired during subsequent 

 growth. 



As long ago as 1851, Lavalle had asserted that, when 

 one solid angle of an octahedron of alum is removed, the 

 crystal tends to reproduce the same mutilation on the 

 opposite angle, when its growth is resumed ! This re- 

 markable and anomalous result has, however, by some 

 subsequent writers been explained in another way to that 

 suggested by the author of the experiment. 



In the same way the curious experiments performed at 

 a subsequent date by Karl von Hauer, experiments which 

 led him to conclude that hemihedrism and other pecu- 



Bull. Geol. Soc. Paris, 2me ser., 

 imps, ii , its3, pp. 454-56 ; Coiitpt. 



,ol. vili. 

 '■end., y> 



3, 1851 ; Moigno, 

 pp. 493-95- 



