VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 551 



Observations. — The apparent insolubility of terra ponderosa aerata in the di- 

 luted vitriolic acid i 2, can be accounted for by remarking, that the moment the 

 surface of the lump was acted on by the acid, an insoluble coat of marmor 

 metallicum was formed on it, which effectually precluded any further operation 

 of the acid. Professor Bergman, in order to obtain the earth from the terra 

 ponderosa vitriolata, directs the latter to be roasted with fixed alkali, and the 

 dust of charcoal ; but I have always done it by charcoal-dust alone, though pro- 

 bably this method may require a greater degree of heat. It has been remarked, 

 that terra ponderosa and calx of lead resemble each other in many respects ; and 

 I must add, that the vitriol of lead dissolves in the highly concentrated vitriolic 

 acid much in the same manner that the marmor metallicum does, and like this 

 too separates on the affusion of water ; but I never observed it disposed to crys- 

 tallize. The marmor metallicum may probably be useful in some cases where a 

 powerful flux is wanted ; for I once mixed some of it with the black flux, and 

 exposing it to a pretty sharp heat, it entirely ran through the crucible. May not 

 therefore some of the more common varieties of it be used advantageously as a 

 flux to some of the more refractory metallic ores ? 



Sect. 3. — Terra Ponderosa Vitriolata. Variety, calk or cauk. Marmor 

 metallicum, Cronstedt Min. § 18. b ? It is plentiful in the mines in Derby- 

 shire. Is of a white or reddish colour ; crystallized in rhomboidal laminae, but 

 these very much intermixed and confused. Loses little or nothing of its weight 

 by being made red-hot. Specific gravity 4.330. 



Exper. a. Ground in a mortar, and washed over, the washing water when de- 

 canted gave no precipitation with mild vegetable alkali ; but with nitre of silver 

 and nitre of mercury the slightest cloud imaginable. 



b. 100 grs. boiled in marine acid weighed, after proper washing and drying, 



99-5- 



c. The acid solution b let fall a Prussian blue on the addition of a single drop 



of phlogisticated fixed alkali ; and, when saturated with mild fossil alkali, afforded 

 an ochry-coloured precipitate. 



d. This precipitate, collected and washed, weighed half a grain. It was 

 roasted with tallow, and then was wholly attracted by a magnet. 



e. A quantity of the cauk, finely powdered, was mixed with charcoal-dust, 

 and roasted in a crucible at a white heat, for 5 hours, fresh charcoal-dust being 

 occasionally added. It gave out a strong smell of sulphur. 



f. To this roasted cauk nitrous acid was added, which dissolved the greater 

 part of it ; producing, during the solution, some effervescence, and a strong 

 smell of hepar sulphuris. 



g. Some of this solution, after proper evaporation, afforded beautiful crystals, 



