1H13] Great Advance in Crystallography 677 



WEEKLY EVENING MEETING, 



Friday, March 14, 19 lo. 



DoxALD W. C. Hood. Esq., C.V.O. M.D. F.R.C.P., Vice-President, 

 in the Chair. 



A. E. H. TuTTON, Esq., D.8c. M.A. F.R.S. 

 Great Advance in Crystallography. 



Crystallography has made such remarkable progress during the 

 last few months, and the position at the present moment is so 

 interesting, that it was considered opportune to review it in a 

 Discourse from this historic lecture -table. For, firstly, the descrip- 

 tions of the crystals of all the ten thousand su])stances which have 

 ever been subjected to goniometrictil measurement have been collected 

 together and classified within the four volumes of a monumental work 

 by Professor von Groth, of Munich. Secondly, this immense labour 

 has been paralleled by the construction, by Professor von Fedorow, 

 of St. Petersburg, of a tabular record of the main crystal elements 

 of all these substances, arranged in a simplified form, and with the 

 assurance, which has entailed untold labolir to achieve, that they 

 relate to a truly comparative orientation ; so that this table is the 

 index to and basis of a new method of " Crystallo-chemical analysis," 

 which enables a trained investigator to identify any well-crystallized 

 substance from the result of a l^rief goniometrical examination. 

 And, thirdly, the whole of these invaluable results have been placed 

 on a firm experimental basis ; for the internal structure of crystals, 

 as imagined in all its wonderful details by the greatest geometrical 

 and mathematical minds amongst us, has been revealed on the 

 photographic plate as the result of direct experiment with the 

 excessively minute and all-penetrating wave-motion, or corpuscular 

 energy, of the X-rays. 



It is easy to prove that a crystal has an organized structure. The 

 fact is at once revealed by the influence of rapidity or slowness of 

 growth on its character. For example, if a little "benzoic acid l)e 

 melted on a glass plate over a spirit lamp and the plate allowed ro 

 cool rapidly in the air, on the object -stage of the projection polari- 

 fecope. using crossed Nicols, the dark field is almost immediately 

 illuminated by crystals beginning to grow at the margin of the liquid 

 film, and from each bright spot a crystal-needle darts, with fiery tip, 

 like a lightning flash, into the centre of the field, until the whole 

 picture is an interlacing mass of acicular crystals brilhantly coloured 



