Mineralogy. -183 



which is again immediately decomposed. In a dry atmosphere they pre- 

 serve their original appearance. No matter is disengaged from it by the 

 application of heat. It is perfectly soluble in water. When reduced to 

 powder, and brought in contact with a small portion of water, it imme- 

 diately combines with it and crystallizes, during which some heat is dis- 

 engaged. It consists of 



Sulphate of soda 99.78 



Subcarbonate of soda 0.22 



The analysis was conducted in the following manner. The sulphuric 

 and carbonic acids were precipitated by nitrate of baryta, the precipitate 

 ignited, and then digested in nitro-muriatic acid, to reduce entirely to the 

 state of sulphate whatever might have been changed into sulphuret by the 

 contact with the filter at an elevated temperature, and to dissolve the caus- 

 tic baryta arising from the decomposition of the carbonate. This portion 

 of baryta was afterwards precipitated by sulphuric acid. Thenardite has 

 been employed by M. Rodas in manufacturing soap, but it is worked like- 

 wise and sold in its natural state — (Annales de Chimie et de Physique, 

 torn, xxxii. p. 308.) 



This substance was for the first time mentioned as a new species in 

 Professor Mohs mineralogy, * and described likewise by Mr Haidin- 

 ger, t though these descriptions were drawn up not from specimens oc- 

 curring in nature, but from crystals obtained from solutions. The natural 

 crystals are frequently much larger than the artificial ones. Those in M- 

 Cordier's possession, and some that were presented to the school of mines 

 in Paris by M. Casaseca, are about half an inch in diameter. They will 

 therefore allow of a much more accurate determination of the specific gra- 

 vity than the crystalline coats whose weight is given in Mohs = 2.462. 

 Their colour is a yellowish-white ; that of the artificial crystal is a pure 

 white, but their transparency is always inconsiderable. Their hardness is 

 = 2.5 of the scale of Mohs, between gypsum and' calcareous spar. 



M. Gay-Lussac has observed that the capacity of water for dissolving 

 sulphate of soda is greatest at 33° Centigr. (106i° Fahr.) Above this tem- 

 perature crystals of Thenardite are deposited, and this is perhaps the con- 

 dition upon which its formation at Espartines depends. 



13. Halloysite, a new mineral species. — Occurs in more or less regular 

 globular masses, sometimes larger than the fist, in those aggregated masses 

 of ores of iron, zinc, and lead, which frequently are found filling the cavi- 

 ties in the neighbourhood of Liege and Namur. It was first observed by 

 M. Omalius d'Halloy, in compliment to whom it has been named by 

 M. Berthier. It is always compact, with a conchoidal fracture some- 

 what resembling wax; it yields to the nail, and is polished by it; its 

 colour is white, generally with a slight bluish-grey tint ; it is translu- 

 cent on the edges ; it adheres to the tongue. It imbibes water like the 

 hydrophane. If exposed to an elevated temperature, it loses in weight, 

 but acquires much hardness, and its colour turns milk-white. 



* Grundriss. vol. i. Translation, vol. ii. p, 33. 

 f Edin. Phil. J own. vol. x. p. 314. 



