62 Proceedings of Philosophical Societies. [Jul1t ? 



a bluish light ; another is transparent ; and the third opaque. 

 The first two varieties give out air, and become transparent when 

 placed in water. The third likewise gives out air, but remains 

 opaque. If the first varieties be only slightly wetted, they 

 become quite opaque. The first two varieties resemble the 

 hydrophanous opal in the property of becoming transparent when 

 put into water, but differ from it remarkably in becoming opaque 

 from the application of a small quantity of that fluid. As taba- 

 sheer disengages more air than the opal, its pores must be more 

 numerous ; and, therefore, the transmission of light, so as to 

 form a perfect image, indicated either a very feeble refractive 

 power, or some peculiarity in the construction of its pores. 

 Dr. B. accordingly found by experiment that this curious sub- 

 stance has a loAver refractive power than any other solid, or even 

 liquid, and that it holds an intermediate place between water and 

 the gases. Numerous experiments were detailed with the view 

 of determining the power of tabasheer to absorb different fluids, 

 and to ascertain the corresponding effects produced upon its 

 optical properties, specific gravity, &c. The paper was con- 

 cluded with an attempt to explain the cause of the paradox 

 exhibited by the first two varieties of tabasheer, of becoming- 

 opaque by a small quantity of water. 



May 13. — A paper, by T. A. Knight, Esq. was read, upon the 

 different qualities of spring and winter felled oak trees. 



The author concluded from his experiments and observations 

 that in all cases where it is essential to give durability to the 

 alburnum of the oak, the tree should be barked in the spring, 

 and felled the ensuing winter. 



At this meeting also, a paper, by Dr. Marcet, was begun, 

 entitled " On the Specific Gravity and Temperature of Sea 

 Waters in different Parts of the Ocean and in particular Seas ; 

 with some Account of their saline Contents." 



May 20. — Dr. Marcet's paper was concluded. The object of 

 this interesting memoir was to determine the general properties 

 of sea waters in different parts of the ocean and at different 

 depths, with the view of ascertaining whether they differed 

 from one another, and in what respect. The investigation was 

 begun several years ago in conjunction with the late Mr.Tennant, 

 and in order to render it as complete as possible, every opportu- 

 nity was taken to collect specimens from all parts of the globe. 

 After these preliminary remarks, Dr. M. proceeded to describe 

 the instruments which had been contrived to raise water from 

 different depths, or from the bottom of the seas when practicable, 

 and which had been employed in collecting the different speci- 

 mens. The three great points kept in view in the course of the 

 examination were, their specific gravity, their chemical compo- 

 sition, and their temperature, when this could be ascertained ; 

 and the author gave a circumstantial detail of the precautions 

 attended to in the investigation of each of these important 

 points. The results of the author's numerous observations and 



