iDlo] on Great Advance in Crystallography 701 



available are not of the high degree of perfection requisite in order 

 clearly to substantiate the last few minutes of any particular angle ; 

 for the differences of angle between the members of series formed by 

 metallic family analogues have been shown by the lecturer to be very 

 mimite, although unmistakable given the most perfect crystals, and 

 have also been found to obey the law of progression according to the 

 atomic weight of the metal. For instance, ammonium zinc sulphate 

 was simply returned by Prof, von Fedorow as a member of the isomor- 

 phous series of monoclinic double sulphates and selenates crystallizing 

 with 6H._,0. Qualitative analysis would be necessary after all, in order 

 to discover the actual member of the series present. Moreover, there 

 are certain features of Prof, von Fedorow's own peculiar version of 

 the theory of crystal structure, such as his idea about pseudo-cubic 

 and pseudo-hexagonal types, and his dealing in consequence with 

 many substances as being deformations of a higher symmetry than 

 they actually show, which to the lecturer appear unnecessary compli- 

 cations likely to discourage the use of the new method. But these 

 defects can, and doubtless will, be eliminated as the method becomes 

 practically applied. 



In conclusion, it must be obvious that a great advance has really 

 now been made in crystallography. For the geometrical theory of 

 crystals as homogeneous structures, based on the 1-4 space-lattices as 

 the grosser structures and the 230 point-systems as the ultimate 

 atomic structures, has been not only theoretically perfected, but 

 proved by direct experiment to be an actual fact, by the epoch- 

 making work of Lane, Friedricb, and Knipping. The descriptions 

 and chemical relationships of all the ten thousand measured substances 

 have been brought together in the great book of Prof, von Groth, 

 and the material further sifted, reduced to correct setting, and 

 arranged according to symmetry and elements by Prof, von Fedorow, 

 in a tabular form immediately available as a reference index for 

 identification purposes, thus providing the material for a true crystallo- 

 chemical analysis. The science of crystallography is thus now 

 placed on a secure foundation, supported equally by mathematics, 

 geometry, and experiment, and its natural data are rendered available 

 for chemists and physicists alike. 



It must be obvious that these significant results place the science 

 in such a strong position that it can no longer be ignored as in the 

 past by both physicists and chemists, and that to both an elementary 

 knowledge of the sul)ject will be imperative and absolutely in- 

 dispensable in future. That crystallochemical analysis will ever 

 replace qualitative chemical analysis is neither to be expected nor 

 desired, even if alone on the ground of the admirable training and 

 experience in chemical operations and principles which chemical 

 analysis affords. But so powerful an aid cannot with impunity be 

 neglected, and the knowledge of crystallography required of the 

 chemist who desires to be armed with the new weapon of research 



