101- 



described tin- appearance of the surface immediately under the newly 

 secreted pus. It was made up of eminences and hollows ; the former 

 consisting of clusters of tortuous blood-vessels, the latter filled with 

 pus. After a few minutes' exposure, the following changes were ob- 

 served : a transparent pellicle covered the surface, under which glo- 

 bules of air made their appearance in different places ; then horizontal 

 auastomozing canals, filled with red blood, were seen to form ; and 

 red spots, which were the termination of perpendicular canals, were 

 observed under the pellicle. Drawings illustrating these appearances 

 accompany the paper. 



After detailing further proofs and illustrations of the above phe- 

 nomena, presented by the surfaces of sores covered with pus, Sir 

 Everard observes, that the carbonic acid originally contained in the 

 tubes is very readily displaced by the blood, in consequence of its 

 disposition to absorb that particular gas which forms so large a pro- 

 portion of its component parts. He thinks that the extrication of 

 carbonic acid is the original cause of the tubularity of pus ; and that 

 the tubes are then filled with red blood, and thus connected with the 

 circulation. The succeeding changes are illustrated by Mr. Bauer's 

 drawing, which the author laid before the Society last year. 



On the Laws which regulate the Absorption of Polarized Light by 

 Doubly Refracting Crystals. By David Brewster, LL.D. F.R.S. 

 Lond. and Edinb. In a Letter addressed to the Right Hon. Sir Jo- 

 seph Banks, Bart. G.C.B. P.R.S. Read November 12, 1818. 

 IPhil. Trans. 1819, p. 11.] 



In examining the polarizing structure of acetate of copper, the 

 author's attention was drawn to certain changes of colour exhibited 

 by its crystal, when exposed in different positions to polarized light ; 

 and as these were independent of the thickness of the plate, and of 

 any analysis of the transmitted pencil, he was induced to regard 

 them as a new affection of light, ascribable to the absorption of the 

 homogeneous tints forming the compound colour of the crystal. Dr. 

 Brewster, therefore, collected a variety of coloured crystals, with a 

 view to examine the phenomena which they presented, when cut at 

 different angles with the axis, and when exposed in different po- 

 sitions to polarized light. The details of this examination are next 

 given; and as the property of transparent bodies, by which they 

 detain and assimilate to their own substance a portion of the rays 

 which penetrate them while the rest are freely transmitted, is re- 

 lated to the axes of double refraction, the author first describes the 

 phenomena presented by crystals of one axis, and then explains the 

 modifications which they undergo when the number of axes is in- 

 creased. 



It appears from these investigations that the colouring particles of 

 crystals, instead of being indiscriminately dispersed throughout their 

 mass, have an arrangement related to the ordinary and extraordinary 

 forces which they exert upon light. In some cases, the extraordinary 



