CONTRIBUTIONS FROM THE CHEMICAL LABORATORY OF 



HARVARD COLLEGE. 



A STUDY OF GROWING CRYSTALS BY INSTAN- 

 TANEOUS PHOTOMICROGRAPHY. 



By Theodore William Richards and Ebenezer Henry Archibald. 



Received January 7, 1901. Presented January 9, 1901. 



Countless observers have watched the growth of crystals under the 

 microscope. As long ago as 1839 attempts were made to study also the 

 birth of crystals, in order to determine in what mariner the new phase 

 makes its entrance into the system. With a microscope magnifying 600 

 diameters, Link * thought he could detect the formation of minute glob- 

 ules at the moment of precipitation, — globules which soon joined and 

 assumed crystalline form. Schmidt, f Frankenheim, $ and especially 

 Vogelsang, § made similar observations some years later, and several 

 more recent accounts of this phenomenon have appeared. Modern investi- 

 gators have been more concerned with the speed of separation from super- 

 saturated or supercooled liquids than with the form of the first separation.! 



Ostwald, in 189 1, accepted the interpretation of these data, which as- 

 sumes that crystallization is always preceded by the separation of an 

 initially liquid phase, consisting of a supersaturated solution of the former 

 solvent in its former solute.** 



This explanation is indeed a plausible one, and undoubtedly holds true 

 in cases like those studied by Schmidt and Vogelsang, where a sub- 

 stance separates at a temperature not far below its melting point, and 

 often where a substance soluble in one liquid is precipitated by the 



* Link, Pogg. Ann., 46, '258 (1839). 



t Schmidt, Lieb. Ann., 53, 171 (1845). 



| Frankenheim, Pogg. Ann., Ill, 1 (1860). 



§ Vogelsang, Die Krystalliten (Bonn, 1875). See Lehmann, Molecularphysik, 

 I. p. 730 (1888). 



|| Gernez, Compt. Rend., 95, 1278 (1882); Moore, Zeits. phys. Chem., 12, 545 

 (1893) ; Friedliinder and Tammann, ibid., 24, 152 (1897) ; Tammann, ibid., 25, 441 ; 

 26, 307, 367, 28, 96; Kiister, ibid., 25, 480, 27, 222; Bogajavlensky, ibid., 27, 585. 



** Lehrbuch, I. 1039 (1891). 



