250 Professor John W. Judd [Jan. 30, 



WEEKLY EVENING MEETING, 



Friday, January 30, 1891. 



Edward Frankland, Esq. D.C.L. LL.D. F.R.S. Vice-President, 



in the Chair. 



Professor John W. Judd, F.R.S. F.G.S. 



The Rejuvenescence of Crystals, 



Vert soon after the invention of the microscope, the value of that 

 instrument in investigating the phenomena of crystallisation began to 

 be recognised. The study of crystal-morphology and crystallogenesis 

 was initiated in this country by the observations of Robert Boyle ; 

 and since his day, a host of investigators — among whom may be 

 especially mentioned Leeuenhoek and Vogelsang in Holland, Link 

 and Frankenheim in Germany, and Pasteur and Senarmont in France 

 • — have added largely to our knowledge of the origin and development 

 of crystalline structures. Nor can it be said with justice that this 

 field of investigation, opened up as it was by English pioneers, has been 

 ic^nobly abandoned to others ; for the credit of British science has 

 been fully maintained by the numerous and brilliant discoveries in 

 this department of knowledge of Brewster and Sorby. 



There is no branch of science which is more dependent for its 

 progress on a knowledge of the phenomena of crystallisation than 

 geology. In seeking to explain the complicated phenomena exhibited 

 by the crystalline masses composing the earth's crust, the geologist is 

 constantly compelled to appeal to the physicist and chemist — from 

 them alone can he hope to obtain the light of experiment and the 

 leading of analogy whereby he may hope to solve the problems which 

 confront him. 



But if geology owes much to the researches of those physicists 

 and chemists who have devoted their studies to the phenomena of 

 crystallisation, the debt has been more than repaid through the new 

 light which has been thrown on these questions by the investigation 

 of naturally-formed crystals by mineralogists and geologists. 



In no class of physical operations is time such an important factor 

 as in crystallisation ; and Nature, in producing her inimitable examples 

 of crystalline bodies, has been unsparing in her expenditure of time. 

 Hence it is not surprising to find that some of the most wonderful 

 phenomena of crystallisation can best be studied— some indeed can 

 only be studied — in those exquisite specimens of Nature's handiwork 

 which have been slowly elaborated by her during periods which must 

 be measured in millions of years. 



I propose to-night to direct your attention to a very curious case 

 in which a strikingly complicated group of phenomena is presented in 



