698 Dr. A. E. H. Tutton [March 14, 



proved up to the hilt that, except in the cases of cubic crystals 

 identical in angles in accordance with their perfect symmetry, 

 every solid crystallizable substance is characterized by its own 

 peculiar crystalline form and interfacial angles. This is quite 

 true, even to the last minute of angular measurement, when the 

 conditions of crystallization are ideal. When thus perfect, even iso- 

 morphous substances show differences among themselves to the extent 

 of a definitely measurable number of minutes. But such perfection 

 of growth is not easy to attain, and, in ordinary crystallization with- 

 out special precaution against disturbance, is rarely found. The 

 essential crystallographic measurements can, however, be made in 

 an hour's time, provided use be made of the two- or three-circle 

 form of goniometer, such as the excellent one devised by Dr. 

 Herbert Smith. This form of goniometer enables all the needful 

 measurements of the interfacial angles to be made with a single 

 setting of the crystal on the wax of the holder. But practical 

 difficulties have hitherto still stood in the way. Excellent as is 

 von Groth's classification — and the most suitable for a work of 

 reference of the full and comprehensive character of this per- 

 manent monuliient of the master's industry and wide knowledge 

 of chemistry, related compounds being arranged and compared in 

 close proximity — the very nature and size of such a work renders it 

 unsuitable for the purpose of discovering rapidly the chemical com- 

 position of a substance from its geometrical elements. An index of 

 substances arranged in the order of their symmetry and the numerical 

 values of the crystal constants within the system is what is needed, 

 and this has now for the first time been drawn up for the ten 

 thousand measured substances by Professor von Fedorow. 



Another difficulty then presents itself. It often entirely depends 

 on how a crystal is held in space, that is, which direction in it is to 

 be the vertical axis, which the right-and-left axis, and which the 

 front-and-back axis, as to what the nature of the crystal constants 

 (elements) will be. Moreover, even if two diiferent observers choose 

 these similarly, they may select a different parametral plane (a fourth 

 face other than the three faces parallel to the axes, and cutting oiT 

 unit lengths from the latter) to determine the axial ratios. Hitherto, 

 beyond a few arbitrary rules — for instance, that the right-and-left axis 

 of a rhombic crystal shall be longer than the front-to-back one — there 

 has been no definite guiding principle for the determination of the 

 setting. Professor von Fedorow has now given us one, by means of 

 which we can be sure which are the real vertical faces (prismatic or 

 pinakoidal), which is the basal plane (the pair of top and bottom 

 faces), and which set of pyramid faces are the important ones fixing 

 the relative axial lengths. The true setting has been determined by 

 Professor von Fedorow for every one of the substances in his index, 

 and the crystal elements for such setting calculated. 



The mode of classification adopted in this index-catalogue is based 



