beautiful examples, while the pretty dendrites exhibited by many 

 substances when crystallising from a solvent, are closely related 

 to the former, and known to all investigators who have studied 

 crystallisation-phenomena through the miscroscope. 



But even if the number of combining individuals is much sualler 

 than in the cases mentioned, definite aggroupments of a few indi- 

 viduals frequently occur, which from a crystallographical point of 

 view have certainly not less importance than the separate crystals 

 referred to in the previous chapters. 



If, in contrast with the special circumstances mentioned above, 



Fig. 132. Snow-Crystals. 



crystallisation occurs in solutions which are only slowly moved, 

 or if the process of crystal-formation is purposely retarted by 

 increasing the viscosity of the solvent by the addition in minute 

 quantities *) of gum or gelatine, then between only a very small 



l) O. Lehmann, Molekularphysik I. p. 415. (1888). 



Here the aggregation-phenomena of crystals of different species are omitted; 

 see for these interesting facts: F. Wallerant, Crystallographie, Paris, (1909). 

 p. 491; J. Beckenkamp, Statische und Kinetische Krystall-theorien I, p. 187. 

 (1913); Th. V. Barker. Journ. Chem. Soc. 89. 1120. (1906); Miner. Magaz. 14. 

 235. (1907); O. Miigge, Neues Jahrb. f. Miner. Beil. Bd. 16. 335. (1903). 



Moreover Grand jean (Bull, de la Soc. Miner. 39. 164. (1916) was able to 

 demonstrate that also the fluid crystals and anisotropous liquids of p-azoxyanisol, 

 p-azoxyphenetol, anisaldazine, ethyl-p-azoxybenzoate, and ethyl-p-azoxycinnamate 



