the native Carbonate of Soda from Fezian 3:3 L 



was united to the sulphate of soda ; while in the analysis by 

 Mr Rivero, the proportion of soda is a little larger than the for- 

 mula would require. 



Klaproth observed, that it does not, like the common crys- 

 tals, dissolve in its water, but that it retains its form, though it 

 be exposed to a moderate red heat. It gives off the water 

 with a crackling noise, if exposed in a glass tube to the spirit- 

 lamp. It is much more difficultly soluble in water than the 

 hemi-prismatic, or also the prismatic natron-salt ; also its taste 

 is less intensely alkaline. It does not like them give off its 

 water of crystallization when exposed to the air ; and it may 

 be preserved for any length of time unchanged in an atmos- 

 phere, rendered perfectly dry by the contact of lime. 



The chemical difference of the prismatic natron-salt and the 

 hemi-prismatic species, if any, probably lies in the quantity of 

 water which they contain, but it has not as yet been ascertain- 

 ed. They were first distinguished from each other as parti- 

 cular species in the first volume of the Grundriss der Miner a- 

 logie, by Professor Mobs, p. 526 ; the hemi-prismatic form of 

 one of the species has also been recognised by Messrs Brooke 

 and Levy. They may be both easily obtained from a solu- 

 tion of carbonate of soda. If this solution be perfectly satu- 

 rated, and exposed to a farther evaporation, at a temperature 

 of about 80° — 100° Fahr., beautiful crystals of the prismatic 

 species will be formed, whilst a less saturated solution will pro- 

 duce hemi-prismatic crystals at a lower temperature, or if cool- 

 ed more rapidly. By recrystallizing under different circum- 

 stances, the crystals of the two species may be easily trans- 

 formed into one another. 



A solution of the supercarbonate of soda of the Edinburgh 

 Pharmacopoeia, exposed to a slow evaporation, yields small 

 transparent crystals, possessing a hemiprismatic character. 

 But they effloresce very readily, and though they seem to be 

 different from those of any of the preceding species, I have 

 not yet succeeded in obtaining them large enough for examina- 

 tion. 



It is not a quite uncommon case, that mineral species which 

 have once been described as such, or at least mentioned in 

 works relating to the science, arc subsequently neglected by 



