( l^c ) 



Ttoceedings of the Boy at Society of Edinburgh, 



(Continued from vol. xxxi. p. 401.) 



1841, December 6. — Sir T. M. Brisbane, Bart., President, 

 in the Chair. The following Communications were read : — 



1. On the Circulation of the Blood, and the Difference of the 



Laws of Fluids moving in Living and Dead Tubes. Part 

 Second. By Sir Charles Bell. 



2. On a Peculiar Structure observed by the Author in the 



Ice of Glaciers. By Professor Forbes. (Printed in this 

 Journal, vol. xxxii. p. 84.) 



December 20. — Dr Hope, V.P., in the Chair. 



1. Report of a Committee on the Papers of David Hume, be- 



queathed to the Society by the late Baron Hume. Com- 

 municated by the Council. 



2. On the Optical Properties of Greenockite, by Sir David 



Brewster, in a letter to Lord Greenock. 



Greenockite has the form of a regular six-sided prism, with py- 

 ramidal summits, the faces of the pyramid being inclined 36° 20' 

 to their base. The pyramids are sometimes truncated on their 

 summit. 



The crystallization is often composite. 



The index of refraction of Greenockite is 2.6882, corresponding 

 to the middle of the green space, and to the ordinary ray. Hence 

 Greenockite exceeds the Diamond m refractive power and also chro- 

 mate oflead^viVx*^ I had found to surpass the diamond in this 

 respect. 



The double refraction of Greenockite is comparatively small, which 

 is not usual in substances of a high refractive power. It is so small, 

 indeed, that owing to its great dispersive power it is exceedingly 

 difficult to separate the two images. 



The polarising angle of Greenockite is 68° 36' for the red rays,^ 

 which corresponds to an index of refraction for that light of 2.5517. 



I found it very difficult to establish the existence of an uniaxal 

 system of rays along the axis of the prism ; but I succeeded in doing 

 this by light of the condensed rays of the sun, by which it can alone 

 be e.stablished ; for when in biaxal crystals one of the axes is very 

 weak, a.s in nitre, its influence on the rays is scarcely visible in 

 crystals of little thickne.'^s, such as those we meet with in Greenockite* 



