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Anniversary Meeting. 



[Nov. 30, 



recognition of the valuable services which you have rendered to your 

 country, as well as to science. 



The Council has awarded a Royal medal to Mr. Thomas Davidson, for 

 his works on the ( Recent and Fossil Brachiopoda,' more especially for his 

 series of monographs in the publications of the Palseontographical Society. 



The publications referred to in this award extend over a period of twenty- 

 three years, viz. from 1847 to 1870 ; the plates which illustrate these 

 works have been drawn by himself upon stone, and freely presented to the 

 different Societies which have published his writings. 



Mr. Davidson, 



I have the pleasure of presenting this Medal to you in testimony of the 

 value which the Royal Society attaches to your writings on British and 

 Foreign Brachiopoda 



The Council has awarded the Rumford Medal to Monsieur Alfred Olivier 

 Des Cloizeaux, for his researches in Mineralogical Optics. 



For nearly thirty years M. Des Cloizeaux has been occupied in investi- 

 gating the characters of crystallized bodies, and more especially those which 

 are produced by their action upon light. He has subjected nearly 500 

 crystalline species to an optical scrutiny, determining the ratio of the velo- 

 city of light of various colours in air to its velocity in the direction of each 

 of the axes of optical elasticity within the crystal, the angle between the 

 optic axes in biaxial crystals, and the dispersion of the optic axes. The 

 results of these observations are published in vols. xi. and xiv. of the 1 An- 

 nates des Mines,' and in vol. xviii. of the ' Memoires des Savants Etrangers 



Assuming the truth of the laws connecting the optical properties of 

 a crystal and its system of crystallization established by Sir David Brewster, 

 he has applied them to correct the descriptions of mineral species, by the 

 methods detailed in his ' Memoire sur l'emploi du microscope polarisant 

 et sur l'etude des proprietes optiques birefringentes propres a, determiner 

 le systeme crystallin dans les cristaux naturels ou artificiels.' Numerous 

 instances of these corrected descriptions occur in the ' Comptes Rendus,' 

 the ' Annales de Chimie,' the 'Annales des Mines,' the 'Transactions of 

 the Royal Society ' for 1868, and in his ' Treatise on Mineralogy.' 



"While investigating the effect of heat in modifying the action of crys- 

 talline bodies, he observed that the positions of the optic axes of felspar, 

 chrysoberyl, and brookite changed on the application of heat, returning 

 to their original positions when the crystal regained its initial temperature, 

 as indeed was known before in the case of felspar ; but on increasing the 

 temperature beyond a certain limit, he made the very unexpected discovery 

 that the positions of the optic axes were permanently altered. Thus in 

 felspar the permanent change occurs at a low red heat, or at about 600° C. 

 M, Des Cloizeaux exhibited this experiment in the presence of the Che- 

 mical Section of the British Association at the Meeting held in Cambridge 



