462 Proceedings of Philosophical Socielies, [June, 



Article XL 



Proceedings of Philosophical Societies, 



ROYAL SOCIETY. 



On Thursday, the 2Sth of April, tlie remainder of Dr. Brew- 

 ster's paper on the optical properties of mother-of-pearl was read. 

 This substance possesses the property of polarizing light in a way 

 different from all other bodies. In crystallized bodies the opposite 

 polarization of the two images is always related to some axis or 

 fixed line in the primitive form ; but no such relation exists in 

 mother-of-pearl. In uncrystallized bodies the reflected pencil is 

 always polarized in an opposite manner to the refracted pencil ; but 

 in mother-of-pearl a single plate possesses the property of polarizing 

 the whole of the transmitted light at an angle of incidence of 60° ; 

 and the transmitted pencil is polarized in the same manner as the 

 reflected pencil. 



At the same meeting a paper by Captain Henry Kater was read, 

 describing a new method of dividing circles, and other astronomical 

 instruments. The method of Bird was used in dividing astronomical 

 instruments till it was superseded by the new method of Troughton, 

 of the perfection of which the mural circle at the Greenwich 

 Observatory exhibits an admirable proof. Captain Kater conceives 

 that the method of Troughton admits of simplification. He con- 

 siders his own method as possessed of the greatest possible simpli- 

 citv, and as very accurate. It would be impossible to render this 

 method intelligible without figures. We shall not therefore attempt 

 to give an outline of it. 



On Thursday, the 5th of May, the remainder of Captain Kater's 

 paper on the method of dividing astronomical instruments was read. 



On Thursday, the 12th of May, a paper by Dr. Benjamin Heyne 

 on the Indian method of oxydizing silver by means of the juice of 

 jatropha cureas, and on the milk of plants, was read. A piece of 

 silver is heated to redness, wrapped in the leaves of any kind of tree, 

 and then quenched in the juice of the jatropha moluccana. This 

 process is repeated about twenty liines, taking care never to fuse the 

 silver. The metal becomes quite brittle, and crumbles to powder 

 between the fingers. Dr. Heyne tried the same process, substituting 

 water instead of the vegetable juice, and a similar effect was pro- 

 duced. From Dr. Heyne's account of the above process I suspect 

 that the silver is not oxidized ; hut merely rendered brittle, and 

 reduced to a fine powder; probably by combining with something 

 which exists in the vegetabjc juice employed, or rather in the cow- 

 dung in which the silver is heated. The only known oxide of silver 

 is a dark gieenish brown powder, which is reduced to the metallic 

 state by a very moderate heat. If the vegctalile juice merely com- 

 municttted oxygen, it is obvious that that principle would be driven 



