146 



The circular polarization of heat by total reflection forms the sub- 

 ject of the ffth section. It had been shevm in the former series 

 that a suitable thickness of mica is capable of producing circular po- 

 larization ; but the more critical experiment of producing this ef- 

 fect by total reflection, in a plane inclined 45° to the plane of posi- 

 tive polarization, remained to be accomplished. This the author 

 succeeded in doing, by using rhombs of rock-salt, or combinations 

 of two prisms of the same substance. (See the Proceedings 

 for 1st February 1836.) When two total reflections at 60° were 

 employed, the appearance of polarization had almost vanished, pro- 

 vided that the planes of reflection and primitive polarization formed 

 an angle of 45°. But if that angle were 0° or 90°, no change was 

 observable in the state of polarization of the emergent light. 



3. Dr Knox exhibited some specimens to prove that the teeth 

 of the Cachalot are devoid of enamel. 



When a longitudinal section of one of these teeth is made, two 

 substances present themselves, viz. a central and a cortical ; these 

 diff"er from each other both in appearance and in structure. The 

 central substance resembles, and is no doubt analogous to the ivory 

 of other teeth, but the cortical exhibits not the slightest analogy to 

 enamel. In texture it is softer even than the ivory portion, and 

 probably continues to grow or be deposited during the greater 

 part of the life of the Cachalot, until it in fact at last completely 

 encloses the central part, which can be nothing but an ossified 

 pulp : it encloses the central portion in the manner that the ivory of 

 the human tooth encloses the soft dentar pulp. 



Sections of a great variety of teeth of the Cetacea were shewn 

 to exhibit a similar structure. 



