226 Proceedings of Philosophical Societies. March, 



Article XIII. 



Proceedings of Philosophical Societies. 



' ROYAL SOCIKTY. 



On Thursday, the 27th of Januarj', a paper by Dr. Brewster was 

 read, on the Polarisation of Light by Refraction. Dr. Brewster 

 has ascertained that when light is transmitted obliquely through any 

 transparent body whatever, part of it suffers polarisation. The 

 quantity of polarised light varies as the cotangent of the angle of 

 incidence ; and there is always a particular angle depending upon 

 the refractive power of the body at which the emergent light is 

 wholly polarised. When the light is transmitted through a number 

 of parallel plates, the cotangents of the angles of polarisation are 

 always to one another as the number of plates employed ; and the 

 number of plates multijilied by the tangent of the angle at which 

 they polarise light is a constant quantity. 



On Thursday, the 3d of February, a paper by Sir Humphry 

 Davy was read, entitled Some further Experiments on Fluorine, 

 and on some other Objects of chemical Investigation. This paper 

 consisted, in fact, of three distinct subjects, treated of in succes- 

 sion. 1. Several further e.Npcriments were related, made in order 

 to obtain fluorine in a separate state, but they were all unsuccessful. 

 Fluorine has such a tendency to enter into combinations, that no 

 vessels can be procured upon which it does not act. Most of our 

 readers are probably acquainted with Sir H. Davy's opinion respect- 

 ing fluoric acid. He considers it as a compound of hydrogen and 

 an unknown supporter of combustion, which he caWsfliior/ne. This 

 fluorine has the property of combining with the base of silica and 

 with boron, and it forms an acid with each. The fluates are com- 

 pounds of fluorine and the metals, which constitute the bases of the 

 salifiable bodies. A number of experiments to determine the pro- 

 portions of the constituents of these bodies were detailed. Fluor 

 spar, when heated with sulphuric acid, increases in weight from 

 100 to 175*4 ; but it was requisite to repeat the process eight times 

 to obtain the full effect. If we suppose fluor spar to be a compound 

 of fluoric acid and lime, this result gives us its composition as 

 follows : — 



Lime 73-C6 1 



Fluoric acid 26-33'.) 



100-000 



But if i'.s constituents arc fluorine and calcium, then its compo- 

 sition is — 



