264- Mr. Phillips on the Crystalline Form of the Gaylussite. 



ments to that of M. Cordier, and altogether constituted of 

 different planes ; that which is adopted by that gentleman does 

 not coincide with the cleavages of the mineral, while that which 

 I propose is bounded by them : the terminal planes decline 

 from one acute angle of the prism to the other. 



The planes e ^' of the following figures have been adopted 

 by M. Cordier as the lateral planes of his primary prism, and 

 the plane c (if I understand his statement correctly), as the 

 terminal plane. — Slight, but very uncertain indications of cleav- 

 age exists in the direction of the latter, but none parallel to 

 the former; while cleavages parallel to the planes M M are 

 easily obtained, and of uncommon brilliancy ; and parallel to 

 P, I have obtained a cleavage sufficiently bright for the use of 

 the reflective goniometer. 



Fig. 1. Fig. 2. Fig. 3. 



Measurements by the rejlective Goniometer. 



M on M on cleav- 

 age planes 

 P on M or M' . 



68° 50' 



. 96 30 



. 49 55 



. 125 10 



.136 32 



. 90 5 



M on c 110 20 



c . . 

 e or e' 

 gorg' 

 k . . 



M on^ 137° 45' 



-g 110 10 



e on e' ..... 



145 



70 



g 152 



k 144 



g on ^ 110 



k 124 



35 

 30 

 20 

 46 

 30 

 30 



The crystals do not occur in the determinate form of fig. 1, 

 but are generally elongated, owing to the increased dimen- 

 sion of the planes g,^, thusgreatly reducing the planes M M', 

 or annihilating them, as in fig. 2 : the ci'ystals are often still 

 further elongated, by narrow and repeated alternations of por- 

 tions of the plane e <?', and g, g', thus giving them the effect of 

 being deeply grooved, or channelled. 



As no description of this mineral has yet appeared, as I be- 

 lieve, in any one of our Journals, I subjoin tlie following, 

 chiefly extracted from the accounts given by MM. Boussingault 

 and Cordier. 



It occurs in detached crystals, disseminated in clay ; the 

 less perfect of them might readily be mistaken for crystals of 

 selenite, — the more perfect and smooth have more nearly the 



aspect 



