1870.] 



President's Address. 



127 



in 1862. This discovery leads to a conclusion of great importance to the 

 geologist. The optic axes of a crystal of felspar serve as the index of a 

 self-registering thermometer, and show that the crystal had never been 

 exposed to a temperature reaching to 600° C. 



Circular polarization was first discovered in quartz by Arago in 1811, 

 and in chlorate of soda, bromate of soda, and acetate of uranium and soda, 

 all three belonging to the cubic system, by Marbach in 1854. In 

 1857 M. Des Cloizeaux detected its existence in cinnabar, producing a 

 rotation of a plane-polarized beam, sometimes to the right, sometimes 

 to the left, to from 15 to 17 times the angle of rotation due to an equal 

 thickness of quartz, — and also, nearly at the same time, in sulphate of 

 strychnine with 13 combining-weights of water, a crystal belonging to 

 the pyramidal system, the rotation, in one direction only (Herschel's 

 right-handed rotation), the same as that of its solution, and about half 

 as great as in an equal thickness of quartz. In February 1869 M. Des 

 Cloizeaux discovered that benzile (C 14 H 10 O 2 ), a compound obtained by 

 Laurent from the essence of bitter almonds in 1835, in crystals belonging 

 to the rhombohedral system, produced a rotation a little greater than that 

 of quartz. The crystals which came first into his possession were all dex- 

 trogyre ; but he has lately obtained some producing a rotation in the op- 

 posite direction. 



In his memoir on the Crystallization and Internal Construction of Quartz 

 he has largely employed the method of exploration by polarized light. 



During the summers of 1845 and 1846 he paid two visits to Iceland, 

 in one of which, conjointly with M. Bunsen, he determined the tem- 

 peratures, at different depths, of the great Geyser and the Strokkur, with 

 the view of discovering the source of their heat, and the cause of the 

 eruptions. 



Any remarks on his very numerous and valuable determinations of the 

 forms of crystalline species, as well as on the geological observations made 

 by him in Iceland, would be obviously out of place in a summary of the 

 researches on light and heat which render him eligible as a recipient of 

 the Rum ford Medal. 



Professor Miller, 

 In Monsieur Des Cloizeaux's unavoidable absence, I request you, as our 

 Foreign Secretary, to convey to him this Medal, in recognition of the value 

 which the Royal Society attaches to his researches. 



Gentlemen, 



If elected for the year which commences this day, and if I should be able 

 to meet you here at your next anniversary, it will be to deliver over this 

 Chair, doubtless to a younger, it may well be to a worthier occupant ; it 

 can hardly be to one having the welfare of the Royal Society more warmly 

 at heart. 



