Ill 



On the Action of Crystallised Surfaces upon Light. By David 

 Brewster, LL.D. F.R.S. Lond. andEdinb. In a Letter addressed 

 to the Right Hon. Sir Joseph Banks, Bart. G.C.B. P.R.S. #c. #c. 

 Read February 25, 1819. [Phil. Trans. 1819,;?. 145.] 



It has been remarked by Malus, that the action which the first 

 surface of Iceland spar exercises upon light, is independent of the 

 position of its principal section ; that its reflecting power extends 

 beyond the limits of the polarizing forces of the crystal : and that 

 as light is only polarized by penetrating the surface, the forces 

 which produce extraordinary refraction begin to act only at this 

 limit. He also remarks, that the angle of incidence at which this 

 spar polarizes light by partial reflection is 56 30' ; and that, what- 

 ever be the angle comprehended between the plane of incidence and 

 the principal section of the crystal, the ray reflected by the first sur- 

 face is always polarized in the same manner. 



Dr. Brewster's experiments detailed in this paper lead him to oppo- 

 site conclusions, and indicate an extension of the polarizing forces 

 beyond the crystals. He also shows that the force of double refrac- 

 tion and polarization emanates from the surface of bodies, though its 

 intensity depends upon the inclination of the surface to the axis of 

 the crystal ; and that the ordinary or extraordinary image may be 

 extinguished at pleasure in any doubly-refracting crystal, which is 

 thus converted into a singly-refracting crystal ; that the change in 

 the angle of polarization, produced by the interior force, depends on 

 the inclination of the reflecting surface to the axis of the crystal, 

 and upon the azimuthal angle, which the plane of reflection forms 

 with the principal section ; and that the change in the direction of 

 the polarization depends upon the angle which the incident ray 

 forms with the axis of the crystal. 



On the Specific Gravity and Temperature of Sea-Waters, in different 

 Parts of the Ocean, and in particular Seas ; with some Account of 

 their Saline Contents. By Alexander Marcet, M.D. F.R.S. #c. 

 Read May 20, 1819. [Phil. Trans. 1819,^. 161.] 



After some preliminary remarks upon the labours of others in this 

 department of inquiry, Dr. Marcet proceeds to the immediate objects 

 of his own investigation, which were to ascertain the specific gravity 

 of many specimens of sea-water from different parts, and afterwards 

 to examine their saline contents. The results relating to the first of 

 these objects are prefaced by an account of the mode of taking the 

 specific gravities, and of the instrument by which the water was 

 raised, and of which two plates are annexed. The author then pro- 

 ceeds to the inferences deducible from his experiments, which, for 

 the sake of conciseness, are given in the form of tables ; whence it 

 appears that the ocean in the Southern hemisphere is rather more 

 salt than in the Northern, in the proportion of 1029-19 to 1027'57. 

 The mean specific gravity of specimens from various parts of the 



