VOL. LXXXI.] PHILOSOPHICAL TRANSACTIONS. Ill 



contain, not ordinary tabasheer, but a solid pebble, about the size of half a pea. 

 Externally this pebble was of an irregular rounded form, of a dark-brown or black 

 colour. Internally it was reddish-brown, of a close dull texture, much like some 

 martial siliceous stones. In one corner there were shining particles which ap- 

 peared to be crystals, but too minute to be distinguished even with the micro- 

 scope. This substance was so hard as to cut glass ! A fragment of it, exposed to 

 the blow-pipe on the charcoal, did not grow white, contract in size, melt, or 

 undergo any change. Put into borax it did not dissolve, but lost its colour, and 

 tinged the flux green. With soda it effervesced, and formed a round bead of 

 opaque black glass. These two beads, digested in some perfectly pure and white 

 marine acid, only partially dissolved, and tinged this menstruum of a greenish 

 yellow colour ; and from this solution Prussite of tartar, so pure as not, under 

 many hours, to produce a blue colour with the above pure marine acid, instantly 

 threw down a very copious Prussian blue. 



p. s. In ascertaining the specific gravity of the Hydrabad tabasheer, ^ 1 (g), 

 great care was taken in both the experiments that every bit was thoroughly 

 penetrated with the water, and transparent to its very centre, before its weight in 

 the water was determined. 



XXIII. A Second Paper on Hygrometry. By J. A. De Luc, Esq., F. R. S. 



p. 389. 



In the first part of this 2d paper, at p. 1 of this volume, Mr. D. treated of 

 the fundamental principles of hygrometry, and of some hygroscopic phenomena ; 

 and this part relates to a particular application of those premises. Since the pub- 

 lication of his first hygrometer, many others have been invented, 2 of which were 

 chiefly in use; the hair hygrometer of M. de Saussure, and Mr. De Luc's hygro- 

 meter made of a slip of whalebone. If the comparative points of those instru- 

 ments could be determined in the whole extent of their scales, the only incon- 

 venience of their being both used would be, the necessity of reducing to one of 

 them, the observations made with the other; but from 70 to 100 of Mr. 

 D.'s, which space includes the most important period of moisture, their cor- 

 respondent indications are as different from each other, and as variable, as 

 if they were the effects of 2 very different causes. Therefore it is important 

 to decide which of them should remain our only measure of moisture, till, if 

 possible, a better one is found. The following pages, Mr. D. hopes, will lead to 

 that decision. 



The fundamental process of M. de Saussure, with the view of discovering the 

 effects of moisture on the hair hygrometer, was this. He repeatedly caused suc- 

 cessive known quantities of water to evaporate into a close glass vessel, previously 

 reduced to extreme dryness, and containing that hygrometer and a manometer ; 



