RECENT PROGRESS IN CRYSTALLOGRAPHY 93 



instrument at the disposal of crystallographers, soon revealed 

 the fact that the members of isomorphous series exhibited slight 

 differences in their angles between analogous faces, which could 

 scarcely be accounted errors of measurement or as being due 

 to slight distortion. Still, no law or regularity was observed 

 among these small differences, and the measurements by different 

 observers had been so various that results of the most conflict- 

 ing kinds were adduced. Indeed, so late as the year 1891, there 

 was insufficient evidence on which to base an irrefragable 

 decision as to whether the interfacial angles of the crystals of 

 any one substance are a specific property of that substance or 

 not. As generally happens, however, in the case of careful, 

 conscientious work, both Hauy and Mitscherlich were sub- 

 stantially right, and it is one of the happiest results of the past 

 fifteen years' work on the part of the writer of this article that 

 the views of these two master-minds are now reconciled and 

 harmonised. 



The accumulations of crystallographic literature consequent 

 on the swift progress of chemistry presented to the student a 

 most perplexing mass of detached data, much of it of merely 

 approximate accuracy, and in which neither law nor order was 

 apparent. The publication by Newlands in 1863 of his law of 

 octaves, however, prepared the way for the development by 

 Mendeleeff in 1869 of the periodic law of the elements, a 

 generalisation which, in the form elaborated by its author in 

 the Faraday Lecture of 1889, is of the most far-reaching import- 

 ance. That its value is appreciated in this country has recently 

 (November 1905) been emphasised by the conferment on the 

 illustrious Russian chemist of the Copley Medal by the Royal 

 Society. It is not too much to say that this law reduces the 

 hitherto chaotic mass of chemical knowledge to order. It may 

 be remarked, however, that its usual form of enunciation, that 

 " the chemical and physical properties of the elements are 

 periodic functions of their atomic weights," conveys perhaps the 

 idea that there is some particular virtue in the property of 

 atomic weight, whereas it is only used as a very convenient 

 property by which to identify an element ; for the point of the 

 generalisation is that all the properties of the elements hang 

 together, and that the periodic progressive arrangement is 

 identical for all — namely, that of an octave of elements in a 

 horizontal row, the series of such octaves or periods when 



