CRYSTALLIZATION. POLARIZATION. 



319 



FIG. 495. 



Thin sections of Granite and other rocks of the more or less regularly- 

 crystalline structure adverted to in the preceding paragraph, also of 

 Agate, Arragonite, Tremolite, Zeolite, and other Minerals, are very 

 beautiful objects for the Polariscope. 



711. The actual process of the Formation of Crystals may be watched 

 under the Microscope with the greatest facility; all that is necessary 

 being to lay on a slip of glass, previously warmed, a saturated solution of 

 the Salt, and to incline the stage in a slight degree, so that the drop 

 shall be thicker at its lower than at its upper edge. The crystallization 

 will speedily begin at the upper edge, where the proportion of liquid to 

 solid is most quickly reduced by eva- 

 poration, and will gradually extend 

 downwards. If it should go on too 

 slowly, or should cease altogether, 

 whilst yet a large proportion of the 

 liquid remains, the slide may be again 

 warmed, and the part already solidified 

 may be re-dissolved, after which the 

 process will recommence with in- 

 creased rapidity. This interesting 

 spectacle may be watched under any 

 Microscope; and the works of Adams 

 and others among the older observ- 

 ers testify to the great interest 

 which it had for them. It becomes 

 far more striking, however, when 

 the crystals, as they come into being, 

 are made to stand out bright upon 

 a dark ground, by the use of the Spot 

 lens, the Paraboloid, or any other 

 form of Black-ground illumination; 

 still more beautiful is the spectacle 



when the Polarizing apparatus is employed, so as to invest the crystals 

 with the most gorgeous variety of hues. Very interesting results may 

 often be obtained from a mixture of two or more Salts; and some of the 

 Double Salts give forms of peculiar beauty. 1 A further variety may be 

 produced by fusing the film of the substance which has crystallized from its 



1 The following directions have been given by Mr. Davies (" Quart. Journ. of 

 Microsc. Sci.," Vol. ii., 1862, p. 128, and Vol. v., p. 205) for obtaining these. 

 " He makes a nearly saturated solution, say of the double Sulphate of Copper and 

 Magnesia; he dries rapidly a portion on a glass slide, allowing it to become hot, 

 so as to fuse the salt in its water of crystallization; there then remains an amor- 

 phous film on the hot glass. On allowing the slide to cool slowly, the particles of 

 the salt will absorb moisture from the atmosphere, and begin to arrange them- 

 selves on the gl^ss, commencing from points. If then placed under the Micro- 

 scope, the points will be seen starting up here and there; and from those centres 

 the crystals may be watched as they burst into blossom and spread their petals 

 on the plate. Starting-points may be made at pleasure, by touching the film 

 with a fine needle, to enable the moisture to get under it; but this treatment 

 renders the centres imperfect. If allowed to go on, the crystal would slowly 

 cover the plate, or if breathed-on they form immediately; whereas if it is desired 

 to preserve the flower-like forms on a plain ground, as soon as they are large 

 enough development is suspended by again applying gentle heat; the crystals are 

 then covered with pure Canada balsam and thin glass, to be finished off as usual. 

 The balsam must cover the edges of the film, or moisture will probably get under 

 it, and crystallization go creeping on." 



Crystallized Silver. 



