25 



rapidly round their middle point. Too strong a heat often bursts 

 the cavities, separating the laminae of the topaz. The volatile fluid, 

 escaping, leaves a crystalline residue; the dense fluid disappears en- 

 tirely, and is probably a condensed gas. In one specimen, the faces 

 of the cavities formerly filled with the volatile fluid, are corroded, 

 as if by a solvent, developing crystalline structure ; an appearance 

 analogous to that which has been observed on tho external surface 

 of topazes. In Section V. the author described crystals embedded 

 in the mass of the topaz, some of which can only be detected by 

 polarized light, as they do not affect the transparency of the crystal 

 by common light. He concluded by describing cavities lined with a 

 doubly refracting crust or shell, with optical and crystallographic 

 axes — a phenomenon which has no parallel in mineralogy. These 

 cavities have the appearance of embedded crystals, but are de- 

 tected by depolarizing a uniform tint with a variable thickness of 

 crystal. 



2. On the Extraction of pure Phosphoric Acid from Bones, 

 and on a New and Anomalous Phosphate of Magnesia. 

 By Dr Gregory. 



The author, after explaining the methods hitherto proposed for 

 purifying the phosphoric acid of bone earth, and pointing out their 

 inconveniences, mentioned that the chief difficulty was the separa- 

 tion of the magnesia always present in bone earth. The lime, it 

 has been for some time known, may be entirely removed by means 

 of sulphuric acid. In repeating the process of Liebig, which did 

 not succeed in his hands, and which requires the use of alcohol, he 

 found that, after the separation of the lime, there is obtained, by 

 evaporation and heating, a clear and colourless glass, containing all 

 the magnesia : that this glass dissolved completely by boiling in 

 water ; but that the solution, if again evaporated and heated to 

 600° for a quarter of an hour in a platinum capsule, became turbid, 

 and deposited the whole magnesia as an insoluble salt, AVhen water 

 was added, so as to dissolve the phosphoric 'acid, and the insoluble 

 salt separated by the filter, tho filtered liquid was found absolutely 

 free from magnesia, and was a solution of pure phosphoric acid. 



The insoluble salt of magnesia above mentioned is new. It is 

 remarkable for its insolubility in water and acids ; and its composi- 

 tion, as ascertained by several concurring analyses, is — 



