THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] 



JANUARY 1901. 

 \** 



I. Crystal Symmetry. The Actual Basis of the Thirty-two 

 Classes. By William Baklow*. 



[Plates I. & II.] 



IN the study of crystals a large part of the evidence as to 

 the nature of the symmetry displayed is not derived from 

 their external form, but from properties of the crystal sub- 

 stance, such as its action on polarized light, its different 

 degrees of vulnerability in different directions when attacked 

 by solvents, &c.f: it may indeed be said that no investigation 

 of the symmetry of one of these bodies which fails to take 

 into account the tests of internal symmetry is now regarded 

 as complete. The additional evidence referred to often seems 

 contradictory to, or difficult to reconcile with, the symmetry 

 of the external form ; in many cases it indicates that what 

 had previously been regarded as single crystals are in fact 

 groups of crystal individuals symmetrically arranged with 

 respect to one another. The difficulty arises from the fact 

 that the grouping of crystals is commonly of a highly sym- 

 metrical character, and such that the face-directions of 



* Communicated by the Author. Published also in Groth's Zeitschrift 

 fur Krystallographie. 



f Comp. Sohncke in IZntwickelung einer Theorie der Krystallstruktur, 

 pp. 3 and 4. This writer gives the following" definition of a crystal :■— 

 " Ein Krystall ist ein homogener fester Korper, dessen geometrisches und 

 physikalisches Gesatnmtverhalten nach den verschiedenen in ihm gezo- 

 genen Richtungen hin im Allgemeinen verschieden ist, und der bei 

 ungestorter Ausbildung Ton ebenen Flachen begrenzt ist.'' 



Phil. Maa. S. 6. Vol. 1. No. 1. Jan. 1901. B 



I 



