Mr. Phillips on the Crystalline Form of the Gaylussite. 265 



aspect of calcareous spar : the latter are colourless and trans- 

 parent, and are doubly refractive in a high degree : in respect 

 of hardness, this substance is between the two above men- 

 tioned. Spec. grav. 1'928, 1*950; but that of the remarkably 

 brilliant and solid crystal above figured, was found by my 

 friend S. L. Kent to be 1 "990. It is extremely brittle ; is ea- 

 sily reduced to a grayish powder; the cross fracture is con- 

 choidal, and the surfaces produced by it are of a vitreous lus- 

 tre. The crystals are neither phosphorescent by friction, nor 

 electric by heat ; nor does any phosphorescence appear if the 

 powder be thrown on a live coal. When exposed to heat in 

 a matrass, it decrepitates slightly and becomes opaque : de- 

 crepitation continues until it has acquired a red heat; if then 

 subjected to the flame of the blowpipe, it melts rapidly into 

 an opaque globule, which once formed, is infusible; and which 

 if placed on the tongue when it is cold, has a decidedly alka- 

 line taste. In nitric acid it dissolves with brisk effervescence, 

 and if then left to spontaneous evaporation, fine crystals of ni- 

 trate of soda are formed, floating in a solution of nitrate of 

 lime. 



This mineral is found in great abundance near Lagunilla, a 

 little Indian village, situated one day's journey S.W. of the city 

 of Merida. It occurs disseminated at the bottom of a small 

 lake in a bed of clay covering carbonate of soda, termed by the 

 natives tirao, which has been described by M. Palacio Faxar, 

 in a Memoir inserted in the first volume of the Institution 

 Journal. The natives term the crystals of Gaylussite clavos 

 (nails), from their general form, doubtless, when greatly elon- 

 gated. A specimen of the it7ao was likewise received by my 

 brother. It occurs in long slender crystals, which are very 

 indeterminate and dull, but affording one bright cleavage pa- 

 rallel to their axis : they radiate from a common centre. 



The following particulars respecting the relative positions 

 of the Gaylussite and Urao are extracted from the letter re- 

 ceived by my brother with the specimens. 



The lake of Laguilla (Lagunilla, Boussingault ; Lalagu- 

 nilia, Faxar) is about two days' journey from the southern ex- 

 tremity of the lake of Maracaibo: it seldom exceeds six feet in 

 depth : the water reposes on a stratum of black slimy nmd in 

 wiiich the crystals of Gaylussite are disseminated. Below the 

 mud, which varies from 18 inches to two feet or upwards in 

 thickness, appears the upper layer of urao, confusedly cry- 

 stallized, and varying from two to four inches in thickness. It 

 is extracted by expert divers, who can remain a long time un- 

 der water; they guide themselves, when diving, by a long 

 pole which they stick into the mud, where they expect to be 



New Series. Vol. 1. No. 4. April 1827. 2 M sue- 



