1096 MICROCKYSTALLISATION. ETC. 



or should cease altogether, whilst a large proportion of the liquid 

 still remains, the slide may be again warmed, so as to re-dissolve 

 the part already solidified, after which the process will recom- 

 mence with increased rapidity. This interesting spectacle may 

 be watched under any microscope, but the instrument specially 

 designed by O. Lehmaim 1 is particularly adapted to studies of this 

 kind. The degree of heat can be varied at will. The phenomena 

 become 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 

 polarising apparatus is employed, so as to invest the crystals with 

 the most gorgeous variety of hues. 



By chemically precipitating crystalline products under the micro- 

 scope we can obtain a still deeper insight into the crystallisation 

 process. One of the earliest workers at this subject was Link,' 2 

 who observed that precipitates first separate in the form of very 

 minute liquid .globules, and that these subsequently coagulate to 

 form an undoubtedly crystalline precipitate. Later investigation 

 of the subject by Fraiikenheim, and then by Vogelsang, 3 led to the 

 conclusion that during the passage of a substance from the dissolved 

 to the crystalline state it passes through a whole series of inter- 

 mediate stages. On allowing sulphur to crystallise very slowly from 

 a carbon bisulphide solution thickened with Canada balsam, the 

 liquid globules, which first separate gradually, solidify to small 

 isotropic spheres termed globulites ; these embryonic forms then 

 coalesce, yielding regular aggregates known as crystallites. The 

 latter subsequently arrange themselves in rows as tnargarites, 

 several of which then amalgamate, forming longulites, and the 

 process of aggregation proceeds until at last the crystalloids the 

 first product in which the structure of the crystal itself is traceable 

 are obtained. The separate existence of so many transition forms 

 has been disputed, notably by Behrens ; 4 but their mention serves 

 the purpose of indicating that the formation of crystalline bodies is 

 really an operation of considerable complexity. 



Upon the temperature maintained during crystallisation depends 

 the size and arrangement of the crystals. Thus santonin, when 

 crystallising rapidly on a very hot plate, forms large crystals 

 radiating from centres without any undulations ; when the heat is 

 less considerable the crystals are smaller, and show concentric 

 waves of very decided form (fig. 815); but when the slip of glass 

 is cool the crystals are exceedingly minute. In the case of cupric 

 sulphate, Mr. B. Thomas 5 succeeded, by keeping the slide at a 

 temperature of from 80 to 90, in obtaining most singular and 

 beautiful forms of spiral crystallisation, such as that represented in 



1 Molekularphysik, 2 vols. Leipzig, 1888 and 1889. 



2 Pogg. Ann. Bd. xlvi. 1839, p. 258. 3 Die Krystalliten, Bonn, 1875. 



4 Die Krystalliten, Kiel, 1874. 



5 See his paper ' On the Crystallisation at various temperatures of the Double 

 Salt, Sulphate of Magnesia and Sulphate of Zinc,' in Quart. Journ. Microsc. 8ci. u.s. 

 vi. pp. 137, 177. See also H. N. Draper on 'Crystals for the Micro-polariscope,' 

 in Intellectual Observer, vol. vi. 1865, p. 437. 



