134 SCIENCE PROGRESS. 



Curiously, however, although scarcely any successful 

 progress has been made towards determining the arrange- 

 ment of the molecules in any actual crystal, some very 

 interesting attempts to determine the relative positions of 

 the atoms within the molecule of certain crystals have 

 recently been made, and deserve consideration as the first 

 efforts to found such speculations upon observed facts. 



APPLICATION OF THE LAW OF GLADSTONE. 



Sollas {Trans. Roy. Dublin Soc, v., p. 157) has at- 

 tempted to interpret the law of Gladstone and Dale, applied 

 to crystallised substances, as indicating- the relative positions 

 of the atoms. 



This law, which states that —7 — is a constant (the 



specific refractive energy) for a given substance whether 

 existing alone or combined with other substances, is in- 

 telligible in the case of liquids or gases, where n denotes 

 the refractive index and d the density ; the specific refrac- 

 tive energy of a compound can then be deduced from that 

 of its components, for the refractive equivalent of the 

 compound {i.e., specific refractive energy multiplied by 

 molecular weight) is the sum of the refractive equivalents 

 of its components. Now, for crystals belonging to the 

 cubic system, both n and d can be supposed to have the 

 same meaning as in the case of liquids, for such crystals are 

 isotropic ; but in other substances the index of refraction 

 varies with the direction of a ray of light within the crystal, 

 and we cannot speak of the refractive index of the sub- 

 stance. Yet, curiously enough, it has been established as 

 an empirical fact that the law holds perfectly for crystals if 

 the mean refractive index be used in the formula. 



Sollas then inquires what is the signification of the 

 formula when the maximum and minimum indices are used 

 respectively in place of the mean index. Thus sodium 

 nitrate crystallises in the rhombohedral system, and has the 

 two following principal indices : — 



Ordinary refractive index = 1 "586. 

 Extraordinary „ = 1*336. 



