60 Analyses of Books. [July, 



Sulphate of barium 51*5 



Fluoride of calcium 48*5 



100-0 



Some error is created by the admixed carbonate of lime ; and 

 which had not been removed. 



This mineral presents us with a remarkable case of combina- 

 tion ; that of a neutral salt with a body which is not a salt, but 

 belongs to an order which is analogous to metallic oxides. I 

 have met with another instance of the same kind. I have exa- 

 mined transparent crystals which were composed of anhydrous 

 sulphate of calcium and chloride of sodium. 



These combinations of their compounds may, however, per- 

 haps, appear to some persons to cast doubts on the opinion that 

 chlorine and fluorine are not acids. 



These compounds will still be deserving of particular attention 

 from consisting of four matters. 



Article IV. 

 Analyses of Books. 



Philosophical Transactions of the Royal Society of Jjondotif 



for 1819, Part IL 



(Concluded from vol. xv. p. 447.) 



Vil. On some new Methods of investigating the Sums of several 

 Classes of irifinite Series. By Charles Babbage, Esq. A.M. 



VIII. On the Optical and Physical Properties of Tabasheer. 

 By David Brewster, LL.D. F.R.S. Lond. and Edin. — The taba- 

 sheer is a concretion in the bamboo, at first gelatinous, but 

 gradually changed into a white soUd. Mr. Smithson first ascer- 

 tained that it consisted chiefly of siHca, and afterwards Fourcroy 

 and VauqueUn analyzed it, and found it a compound of 70 silica 

 and 30 potash and lime. Dr. Brewster found that when the 

 tabasheer is immersed for a short time in water, it becomes 

 opaque ; but a long emersion renders it transparent. This 

 curious property he found to depend upon the uncommonly 

 small power which the tabasheer has to refract light. The absolute 

 refractive power of air being 4530, he found that of the tabasheer 

 to be only 976* 1 . This is on the supposition that the specific 

 gravity of the tabasheer is 2*4. If we were to reckon it only 

 0-66, as has been suggested by a member of the Royal Society, 

 the anomaly would in a good measure disappear; and this 

 would approach the specific gravity if we were to take into con- 



