342 PROCEEDINGS OF THE AMEHICAN ACADEMY. 



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 mangauous sulphate forms at first two liquid phases when 

 alcohol is added to its aqueous solution. On the other hand, the separa- 

 tion 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. It is hard to guess where the 

 line between probability and improbability should be drawn. 



Ostwald has shown that an exceedingly small particle of solid is capa- 

 ble of starting crystallization,* — a fact which may not be wholly foreign 

 to the present discussion. 



In any case, the matter seemed worthy of further experimenting. Ost- 

 wald says, "Die erste Bihlung der Krystalle liisst sich bei Salzlosungen 

 and dergleichen microscopisch nicht verfolgen, weil gewohnlich im Ge- 

 sichtsfelde an einer bislang gleichformigen Stelle plotzlich ein Krystall- 

 chen 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 at- 

 tending 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 mixture at the point of crys- 

 tallization. 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 difficulty is increased 

 by the fact that most crystals are so transparent as to absorb but little 

 light, and reflection is possible only in certain directions. Hence it is 

 hard to obtain a distinct image even in a strong liiiht. Moreover, the 

 machinery necessary for shifting the plates must be so frictionless in con- 

 struction, and so firmly fixed, as to impart no vibration to the camera or 

 the mobile subject of study. 



These difficulties were at least partially overcome by two different ar- 

 rangements, the first of which caused the successive impression of a brighl 

 image in a dark field, and the second registered dark images in a succes- 

 sion of bright fields. Obviously the former was the more economical as 



* Ostwald, Zeits. pbys. Chi m . 22. 289 (1897). 



