346 SCIENCE PROGRESS 



atoms in solids, indeed in liquids also, is always in a great degree 

 regular and definite. 



The most striking evidence supporting this conclusion that 

 liquids and solids should in this respect be classified together is 

 afforded by the recent discovery of liquid crystals, to which 

 attention will presently be directed. 



The term amorphous, in fact, must not be taken to imply 

 irregularity of structure of a substance thus designated ; it 

 merely indicates that the orderly arrangement which obtains is 

 uniform throughout small patches only or at least is not such 

 that any distinguishable physical property is found to vary 

 according to the direction in which it is determined. The 

 definite orderly arrangement of some kind thus everywhere 

 prevailing will, it is obvious, be attributed to the interaction of 

 atomic forces as they attain equilibrium: whenever the conditions 

 are sufficiently simple, as in crystals, equilibrium of similar sets 

 of interacting forces evenly distributed throughout space is 

 productive of homogeneity of structure. The test for the 

 presence of this homogeneity is obedience to Haily's law of 

 rationality of indices. 



The very important fact has long been known that crystals 

 vary both in physical and geometrical properties according to 

 the chemical composition and constitution of the material from 

 which they are built up, this being most clearly shown in 

 connection with those prepared from a variety of substances in 

 the laboratory. 



The sustained labours of a number of crystallographers and 

 other mathematicians resulted during the nineteenth century in 

 the attainment of a remarkably complete geometrical theory of 

 crystal structure. As already intimated, the foundation of this 

 theory was laid by Hauy's discovery ; gradually perfected by a 

 succession of mathematicians, it required more than half a century 

 to reach finality. 



The high degree of perfection of the theory, even before it 

 had been finally completed by the later work of Sohncke, Barlow 

 and Federow, is demonstrated by the following words of Stanley 

 Jevons in his Principles of Science, published in 1879 : 



" Perhaps the most perfect and instructive instance of classi- 

 fication which we can find is furnished by the science of crystal- 

 lography. The system of arrangement now generally adopted 

 is conspicuously natural and is even mathematically perfect. 



