MOLECULAR ARCHITECTURE 347 



A crystal consists in every part of similar molecules similarly 

 related to the adjoining molecules and connected with them by 

 forces the nature of which we can only learn from their apparent 

 effects. But these forces are exerted in space of three dimensions, 

 so that there is a limited number of suppositions which can be 

 entertained as to the relations of these forces." 



Meanwhile many chemists had appreciated the importance 

 of considering crystal structure in connection with the atomic 

 theory ; efforts were made from time to time to unravel the 

 connection subsisting between crystal structure and chemical 

 constitution but the vast accumulation of experimental details 

 bearing on this connection remained uninterpreted at the close 

 of the century. It has been left to the twentieth century to 

 generalise from this rich collection of materials and to employ 

 the crystal measurements of a generation of crystallographers 

 and chemists in the discovery of the fundamental conditions 

 and principles with which the sciences of chemistry and 

 crystallography have to deal. 



A comprehensive account of the accepted mathematical 

 theory of crystal structure, including the history of its develop- 

 ment, is to be found in the Report of the British Association 

 for 1901 (Glasgow), p. 297. A homogeneous structure whose 

 geometrical properties are found to coincide with those of 

 crystals was ultimately defined as follows : " A homogeneous 

 structure is one in which every geometrical point has an 

 environment identical with that of an infinite number of other 

 such points distributed throughout the structure supposed 

 indefinitely extended." The number of types covered by this 

 definition is 65 but when the variety introduced by dis- 

 criminating between structures that are identical with their 

 mirror images and those that are not is taken into account the 

 number of distinct types increases to 230. 



The properties of crystals which guided mathematicians in 

 discerning the extensions and limitations of the fundamental 

 principle of homogeneity that were necessary in order to make the 

 theory coincide with the phenomena may be roughly described 

 as (a) the various physical properties which in so many crystals 

 vary regularly with the direction in which they are determined — 

 the anisotropic condition of crystalline bodies thus variously 

 revealed manifestly indicates the definite and orderly arrange- 

 ment of the ultimate parts ; (b) the constancy of the angles 



