396 



SCIENCE PROGRESS 



acid obtained by Mr. Shearer for Sir William Bragg are given 

 in the latter's paper on organic crystals. 



The study of inorganic crystals is bringing home to us every 

 day that there are a few underlying principles which, if 

 thoroughly understood and properly applied, may in time 

 enable us to arrive at the nature of any crystal-structure, if 

 not by theoretical reasoning alone, at least by theory assisted 

 by a maximum of one or two X-ray observations. Such a 

 principle is that mentioned by Sir W. H. Bragg in his paper 

 on the crystal-structure of ice {Proc. Phys. Soc, vol. xxxiv, April 

 1922). In brief, this " polar principle," as it may be called, 

 states that in that class of crystals in which the molecules are 

 broken into simple positive and negative ions, the structural 

 arrangement is such that each 'positive surrounds itself with 

 negatives, and each negative by positives. The relative 



Fig. 4. — X-ray drawings : naphthalene (top) and phthalic acid (bottom). 



numbers of the ions of each kind are governed by valency 

 considerations. In these crystals the individuality of each 

 molecule seems to disappear ; in fact, we have the old pheno- 

 menon of ionic dissociation in a new form, not, as in solutions, 

 just a haphazard arrangement, statistically neutral, but an 

 orderly arrangement of ions defined by certain fundamental 

 laws of symmetry and valency. For instance, in sodium 

 chloride, each positive ion is surrounded by six negative ions 

 and each negative ion by the same number of positive ions. 

 In fluorspar, each calcium atom, or rather ion, is surrounded 

 by eight fluorines, and each fluorine b}^ half that number of 

 calciums. In the same way, though the process becomes 

 more difficult to work out, the principle can be extended to 

 elements of higher valency, and still holds good in such cases 

 as magnetite and ruby. In the light of our maturer knowledge 

 of these things, it is well conceivable that the complete struc- 

 ture of the alkali halides might have been deduced from this 



