356 SCIENCE PROGRESS 



idea of points with the chemist's notions of the atomic occupation 

 of space. 



The first step towards the solution was taken by William 

 Barlow in 1883.^ In his paper of that date he develops the idea 

 of representing the atoms by spheres and so pictures crystal 

 structures as close-packed arrangements of spheres ; he also 

 points out that most binary compounds, i.e. those which consist 

 of but two kinds of atoms, crystallise in the cubic system and 

 that such compounds may therefore be represented by close- 

 packed homogeneous arrangements of equal spheres. Barlow 

 however, was, at this time unable to extend his ideas in an 

 adequate manner owing to the lack of a clue to the relationship 

 between the sizes of the atomic spheres of different elements. 



In the Report of the British Association for 1901, which has 

 been already referred to, there is an article summarising the 

 state of the knowledge of the structure of crystals at that time. 

 At the conclusion of this report the author states that there are 

 three questions to be answered before the great problems of 

 crystal architecture can be unravelled: (i) What are the parts 

 of which a crystal consists ? (2) How are they arranged ? (3) 

 Why are they arranged in this particular wa}^ ? 



The second question at that time could be regarded as having 

 received a general answer ; the parts of a crystal, whatever they 

 may be, must be arranged according to one or other of the 230 

 types of symmetry. We are now in a position to answer the 

 remaining two questions with a fair degree of certainty that the 

 answer is correct so far as it goes. The solution of the problem 

 is furnished by the joint work of Barlow and Pope with which 

 we may next deal. 



The Barlow-Pope Theory 



In their paper published in 1906 under the title "A Develop- 

 ment of the Atomic Theory which Correlates Chemical and 

 Crystalline Structure and Leads to a Demonstration of the 

 Nature of Valency," ^ Barlow and Pope have interpreted the 

 results of Sohncke and his successors in quite a new light ; they 

 have put forward a treatment of these earlier results which 

 furnishes a concrete interpretation of the chemical idea of the 



' Nature, Dec. 20 and Dec. 27, 1S83. 

 ■ J.C.S. Trans. 1906, p. 1675. 



