172 



any exception in the behaviour of the individuals which have 

 a part in it. 



However, on closer examination, this appears to be by no means 

 the case under all circumstances. In this well-governed society, 

 with its clear lines of demarcation and its strictly defined distinctions 

 of classes and systems, there are a number of individuals which 

 certainly behave rather strangely. 



In the present chapter we propose to deal with some of the 

 phenomena indicated, and as experience has taught, that even these 

 crystallographical eccentricities conform to certain well determined 

 laws, let us see first what remarks and attempts at an explanation 

 of the peculiar behaviour of these individuals have been made up 

 till now. 



2. Since the time of Rome de 1'Isle and Hauy, it has been 

 stated by several observers that many crystals have a strong tendency 

 to form more or less complicated aggregates. This fact has been met 

 with innumerable times especially among minerals, and of some 

 of them this tendency seems to be so characteristic, that separate 

 crystals of such minerals must be reckoned among the greatest 

 mineralogical rarities. 



Whether crystals will deposit from a solution as separate indivi- 

 duals, or as a confused aggregation of irregularly situated crystals, 

 seems to be determined by special circumstances of deposition- 

 velocity, and by the fact whether crystallisation may take place 

 quietly or not. Indeed, if the solution be rigorously stirred or irre- 

 gularly moved during the process of crystallisation, an aggregate 

 of small crystals directed towards all sides is commonly the result. 

 Such crystal-aggregates may be completely irregular: a heap or a 

 crop of arbitrarily grown needles or plates will be present, and no 

 definite relation whatsoever will exist between the mutual orienta- 

 tion of two contiguous individuals. Often, however, a great number 

 of small crystals, if they are even only rudimentary or embryonic 

 (trichites, crystallites, etc.), -- combine in a perfectly regular way, 

 according to definite laws. In such cases more or less symmetrical 

 groupings result, of which the wellknown snow-crystals J ) (fig. 134) 

 are most beautiful examples, while the pretty dendrites exhibited 

 by many substances when crystallising from a solvent, are closely 



!) G. Nordenskjold, Bull, de la Soc. Miner., 16, 59, (1893); 17. 141, (1894); 

 L. Bombicci, Bull, de la Soc. Miner., 3, 85, (1880). 



