Growing Crystals by Instantaneous Photomicrography . 489 



Ostwald, in 1891, accepted the interpretation of these data, 

 which assumes that crystallization is always* preceded by the 

 separation o£ an initially liquid phase, consisting of a super- 

 saturated solution of the former solvent in its former solutef. 



This explanation is indeed a plausible one, and undoubtedly 

 holds true in cases like those studied by Schmidt and Vogel- 

 sang, where a substance separates at a temperature not far 

 below its melting-point, and often where a substance soluble 

 in one liquid is precipitated by the addition of a consolute 

 liquid in which the substance is insoluble. For examples, 

 phenol always separates from aqueous solution in the form of 

 a liquid, and manganous sulphate forms at first two liquid 

 phases when alcohol is added to its aqueous solution. On the 

 other hand, the separation of a high-melting salt like baric 

 chloride from its solution in pure water is much less likely to 

 take place in this way. The admixture of water necessary 

 to lower a melting-point from 900° to 25° would be so large 

 as to make the new phase, a solution of water in baric 

 chloride, supersaturated to an improbable extent. Moreover, 

 we have no evidence of the existence of vitreous baric chloride 

 at low temperatures. 



It has long been known that an exceedingly small particle 

 of solid is capable of starting crystallization ± — a fact which 

 may not be wholly foreign to the present discussion. 



In any case, the matter seemed worthy of further experi- 

 menting. Ostwald says : — " Die erste Bildung der Krystalle 

 lasst sich bei Salzlosungen and dergleichen microscopisch 

 nicht verfolgen, weil gewohnlich im Gesichtsfeide an einer 

 bislang gleichfdrmigeE Stelle plotzlich em Krystallchen 

 erscheint/' While this is true as far as the human eye is 

 concerned, instantaneous photography, an art unknown in 

 Link's time, seemed peculiarly fitted for the unprejudiced 

 recording of the circumstances attending the genesis of 

 crystals. An attempt in this direction is described below. 



The problem resolved itself into the taking of a number of 

 successive instantaneous microphotographs of a suitable mix- 

 ture at the point of crystallization. This problem presented 

 some difficulties, however. In order to secure a sufficiently 

 brief exposure, very great illumination is needed. The 

 greater the magnifying power of the lenses of the microscope- 

 camera, the more intense must be the source of light. The 



* This is obviously not the case in crystallization of water or of a 

 metal, in freezing.— Eds. Phil. Mag. 

 t Lchrbuch, i. p. 1039 (1891). 

 \ Ostwald, Zeits, plnjs. Chem. xxii. p. 269 (1897). 



