173 



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 smaller 

 than in the case 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. 134. Snow-Crystals. 



crystallisation occurs in solutions which are only slowly stirred, 

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

 increasing the viscosity of the solvent by the addition in minute 

 quantities l ) of gum or gelatine, then aggregates only make their 

 appearance between a very small number of individuals. These 



J ) O. Lehmann, Molekularphysi k , 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, Grandjean [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 



