552 PHILOSOPHICAL TRANSACTIONS. [_ANNO 1784. 



not deliquescent, exactly resembling those obtained from the marmor metalli- 

 cum, (§ ii, h.) 



h. To other portions of the solution f, were added fixed vegetable and fossil 

 alkalis, also volatile alkali, each of which precipitated the earth from the acid. 



i. This earth, after exposure to a white heat for one hour, became caustic, 

 and made lime-water, similar in properties to that mentioned at § 1st e. 



k. Some of the part not acted on by the nitrous acid f, dissolved entirely by 

 boiling in highly concentrated vitriolic acid, and wholly separated again by the 

 affusion of water. More water was added, and the whole was boiled again ; but 

 the filtered liquor gave no signs of precipitation on the most liberal addition of 

 mild fixed vegetable alkali. 



It appears therefore, that 100 parts of Derbyshire cauk contain, of marmor 

 metallicum gg.5, calciform iron 0.5. And it is probable, that the redder pieces 

 contain a little more iron. 



Sect. 4. — Terra ponder osa vitriolata. Variety, radiated cauk ; gypsum crys- 

 tallisatum capillare; Cronstedt Min. § 19 B. It was from Pennely by the Bog, 

 near Minsterley, in Shropshire, 1 5 miles from Salop, on the road to Mont- 

 gomery. 



It is somewhat glossy like satin ; yellowish-white, opaque; composed of slender 

 spiculse set close together, and pointing from a centre. In some pieces there are 

 concentric circles of a semi-transparent horn like appearance. It is not very 

 brittle ; may be shaved with a knife ; loses little or nothing of its weight by being 

 made red-hot. Its specific gravity 4.000 ; but after soaking one night in the 

 water 4.200, or more. 



Exper. When treated in the same manner that the Derbyshire cauk was, in 

 the preceding section, 100 parts of it appeared to contain, of marmor metalli- 

 cum 977, calciform iron 2.3. 



Suspecting that the presence of so small a proportion of iron could hardly oc- 

 casion the whole of the apparent differences between the Shropshire and Derby- 

 shire cauks and the marmor metallicum ; and thinking it not improbable that 

 they might contain lead ; I mixed some of them with charcoal-dust and borax, 

 but could not by means of the blow-pipe produce any metallic appearance, though 

 vitriol of lead, treated in the same manner, was readily reduced. I then mixed 

 4 parts of cauk with 1 part of vitriol of lead ; the lead could still be reduced, 

 though not so readily as before. 



General Observations. — The terra ponderosa seems to claim a place between 

 the earths and the metallic calces. Like the former, it cannot be made to assume 

 a metallic form ; but, like the latter, it may be precipitated from an acid, by 

 means of phlogisticated alkali. In many of its properties it much resembles the 



