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



far back as 1830, and rediscovered by Gadolin in 1867, but tbe 

 actual grouping of crystals into tbese classes is of comparatively 

 recent date ; for although prominence is given to it in Miers' 

 Mineralogy, published in 1902, and in Lewis, Crystallography, 

 published in 1899, it is not taken into account in Story-Maskelyne's 

 Morphology of Crystals, published in 1895. 



Now space-lattices only sufficed to represent the symmetry of the 

 most symmetrical (holohedral) class in each system. It has been 

 the work first of Sohnke and later of Schonflies, Fedorov, and 

 Barlow to show how all the thirty-two types of symmetry recognized 

 in crystals can be represented by point-systems, which are generally 

 reducible to a certain number of interpenetrating space-lattices in 

 each case of the same kind. The kind of space-lattice determines 

 tbe system, but the mode of interpenetration of several lattices of 

 the same kind determines the particular class in that system. In 

 setting up a crystal, crystallographers have now in most cases to 

 choose as axial planes three faces which they have reason to suppose 

 are parallel to three sides meeting in a corner of the elementary cell 

 of the space-lattice. Recently Fedorov, by choosing crystallographic 

 axes somewhat on these lines, has prepared a general table of the 

 characteristic angles of all measured crystals, which makes it 

 possible to identify quickly from its crystals any substance that has 

 been previously measured. 



The question now arose as to the nature of the units involved in 

 the crystal structure. The old notion that the crystal-molecule 

 consisted of a polymerization of many chemical molecules was soon 

 discarded, and, through such a work as that of Tutton, the idea was 

 gaining ground that the units in the crystal structure might be 

 single molecules or even the atoms themselves, when Barlow and 

 Pope brought forward their well-known valency theory, according to 

 which the crystal structure of any chemical compound can be 

 explained by the " close-packing" of spheres representing the atoms 

 of its molecules, the sizes of the spheres being approximately pro- 

 portional to the valencies. This theory has been applied with a fair 

 measure of success to many organic compounds as well as minerals, 

 though the recent demonstration, by means of X-ray experiments, 

 of the equality in volume of the elementary cells of the space- 

 lattices of crystals of ammonium sulphate and rubidium sulphate,, 

 would appear to suggest that some modification of it may be 

 necessary which shall take into account other properties of the atom 

 besides the valency. 



Quite recently, as all the world knows, actual experimental proof 

 has been forthcoming of the space-lattice arrangement in crystals. 

 To Laue first came the truly brilliant inspiration that, just as 

 a closely ruled (1,000 lines to the inch) diffraction grating behaves 

 towards light with its wave-lengths measured in thousandths of a 

 millimetre, so it was conceivable would the parallel planes of atoms, 

 as arranged in interpenetrating space-lattices in a crystal, behave as 

 diffraction gratings to X-rays with their wave-lengths (of the order 

 10~ 9 cm.) so infinitely more minute than those of light. The 

 experimental testing of this stupendous idea was marvellously 



