Br. G. T. Prior — Progress of Mineralogy. 15 



successful. Photographic plates exposed to X-rays which had 

 traversed plates of crystals showed spots of maximum effect which 

 were distributed in accordance with the symmetry of the crystal. 

 In the hauds of W. H. Bragg and W. L. Bragg a modification of 

 this method of experiment has given results, the most reasonable 

 interpretation of which fixes the actual arrangement in space of the 

 atoms in crystals of many minerals. In crystals of common salt, for 

 example, the experimental data indicate that the atoms of sodium 

 and chlorine are each distributed on a separate space-lattice of which 

 the elementary cell is a centre-faced cube (i.e. a cube with points 

 not only at the corners but also at the centres of faces), the two 

 lattices interpenetrating in such a way that the sodium and chlorine 

 atoms occur alternately at equal distances apart. In another cubic 

 mineral, iron pyrites, the three atoms of its molecule are again 

 distributed on centre-faced cube lattices, but the interpenetration of 

 the three lattices is so much more complex as to account for the fact 

 that whereas salt crystallizes in the most symmetrical holohedral 

 class, iron pyrites belongs to the less symmetrical pyritohedral class. 



With the account of this brilliant consummation this review may 

 well come to a close. Limits of space allow of a reference only to 

 the wonderful series of experiments by van't Hoff and his pupils on 

 the Stassfnrt salt deposits, which have shown how their minerals 

 separated from solution ; and to the investigations by Yogt, Doelter, 

 and the band of energetic workers in the Geophysical Laboratory at 

 Washington on the problem of the crystallization of silicates and 

 sulphides from fused magmas. These researches which are still in 

 progress are of great geological interest, as they may be expected to 

 throw much light upon the origin of rocks and ore deposits. Other 

 recent investigations, also having their appeal to geologists, are the 

 determinations by Fenner of the temperatures of inversion of quartz, 

 tridymite, and cristobalite, which enable these silica-minerals to he 

 nsed as geological thermometers; and the use made by Joly, Strutt, 

 and others of radioactive work on the rate of disintegration of 

 unstable elements such as uranium and thorium in order to measure 

 geological time, and thus to help towards the solution of the vexed 

 question of the age of the earth. Many of these researches, 

 however, as well as the attempts, both experimental and theoretical, 

 made by Clarke, Tschermak, and others, to elucidate the chemical 

 constitution of silicates, have been the subject of some controversy; 

 and none have as yet reached such a climax as the demonstration of 

 the atomic structure of crystals. 



This last attainment, it may be noted, has been the result of a 

 gradual extension of investigation from great things to the infinitely 

 small. The science of geology began with the study of rocks, the 

 large bodies which constitute the earth's crust. In order to 

 determine the composition of rocks geology had to seek the aid of 

 mineralogy. The latter science, in its turn, after concerning itself 

 at first mainly with the natural history characters of minerals, had 

 to apply for knowledge of their elemental composition to the science 

 of chemistry which it had itself originally created. Finally physics 

 and mathematics had to he invoked in order to explain how the 



