1818.] Proceedings of Philosophical Societies. 143 



It agrees with the genus hirudo, in the body of the animal 

 being furnished with a series of rings ; in locomotion being 

 effected by the alternate attachment of the head and of the tail, 

 and in the existence of one stomach divided into several lateral 

 cells or partitions. It differs from the same genus in the mouth 

 being furnished with a projecting tubular tongue, and in having 

 an abdominal pouch, or cavity, for the reception of the young. 



Article XV. 



Proceedings of Philosophical Societies. 



ROYAL SOCIETY. 



Jan. 8, 1818. — A paper of Dr. Brewster's was commenced, 

 u On the Laws of Polarization in regularly crystallized Bodies ; " 

 and on the 15th, the reading of Dr. Brewster's paper was con- 

 cluded. 



In the introduction to this paper, Dr. Brewster gave a short 

 account of the history and present state of the subject of double 

 refraction and polarization. Malus had examined, with great 

 care, the double refraction of calcareous spar, quartz, arragonite, 

 and sulphate of baiytes, and maintained that he had demon- 

 strated the perfect identity of their action upon light ; whereas 

 it has been shown by Dr. Brewster that the two first have one 

 axis, while the two last have two distinct axes of double refraction. 

 The able researches of M. Biot were made with calcareous spar, 

 rock crystal, beryl, phosphate of lime, tourmaline, feldspar, 

 arragonite, topaz, sulphate of lime, sulphate of barytes ? sulphate 

 of strontian, and mica ; and the result of his experiments in these 

 crystals was, that all of them had only one axis of double refrac- 

 tion and polarisation, excepting certain specimens of mica which 

 had two axes. Dr. Brewster has, however, shown that no fewer 

 than six of these crystals, including sulphate of lime itself, have 

 two axes of double refraction and polarisation. 



In order to determine the laws of polarisation and double 

 refraction, Dr. Brewster examined no fewer than one hundred 

 and eight i/ crystals, in 160 of which he found the property of 

 double refraction. Only twenty-two of these possess one axis, 

 while about eighty possess two separate axes of double refrac- 

 tion ; and since the experimental laws of double refraction and 

 polarisation have been investigated only for crystals with one 

 axis, the general laws of the phenomena remained undetermined. 

 In the course of this paper, Dr. Brewster has proved that there 

 is a constant connexion between the primitive forms of crystals, 

 and the number of their axes, so that the latter may be predicted 

 from the former, and that these axes are coincident with some 



