ON THE STRUCTURE OF CRYSTALS. 297 



TJte Structure of Crystals. — Report of the Committee, coiisistmrj of 

 Professor N. Story Maskelyne {Chairman), Professor H. A. 

 MiERS (Secretary), Mr. L. Fletcher, Professor W. J. Sollas, 

 Mr. W. Barlow, Mr. G. F. Herbert Smith, and the Earl of 

 Berkeley, apiminted to report on the Present State of our Knoic- 

 ledye concerning the Structure of Crystals. (Draicn vp hy Mr. 

 Barlow and Professor Miers, assisted by Mr. Herbert Smith.) 



Part I. 



Report on the Development of the Geometrical Theories of 

 Crystal Structure, 1666-1901. 



The problem of the structure of a crystal presents itself under two aspects ; 

 it involves the consideration (1) of the material which constitutes the 

 crystal, and (2) of the manner in which this material is put together. To 

 the first part of the inquiry belong all speculations and observations which 

 relate to the nature of the crystal unit : as to whether it be a chemical 

 molecule or an aggregation of chemical molecules ; what may be its 

 dimensions and regularity or irregularity ; and what forces co-operate to 

 fix its position and orientation. 



It miglit reasonably be supposed that this part of the inquiry should 

 precede that which relates to the arrangement of the material. In reality, 

 however, very little is known about the actual nature of the ultimate 

 particles of matter in the solid state, and much more is known about the 

 manner in which it must be arranged. For, as the study of crystals has 

 progressed' it has been found that their morphological and physical 

 regularity results from the fact that they are homogeneous ; both the law 

 of rational indices, which regulates the disposition of the faces of a 

 crystal, and the seolotropism, which regulates its physical characters, are 

 in harmony with the geometrical properties of a homogeneous structure. 



Now the distribution of the material in a homogeneous structure may 

 be studied as a geometrical problem quite independently of the nature of 

 the material, for it may be treated as the problem of the homogeneous 

 partitioning of space (see below, p. 310). 



The present portion of the report, therefore, deals exclusively with 

 the geometrical theory of the homogeneous partitioning of space, or (what 

 comes to the same thing) the homogeneous repetition of identical parts in 

 a uniform structure ; a side of the subject which seems to have reached 

 something like finality. 



A second part will be concerned with the nature of the ultimate par- 

 ticles and with the possible arrangements corresponding to actual 

 substances, a side of the subject which presents considerable difficulty and 

 may be said to be still in its infancy. 



In order to put before the reader a clearer and more satisfactory idea 

 of the present state of our knowledge, the historical development of the 

 subject is sketched below, and the more important contributions to this 

 development are discussed in detail. It will thus be perceived that con- 

 tinual progress has been made towards a clearer comprehension of the possible 

 ways in which the homogeneous repetition of parts may take place, each 



