410 l'HILOSOl'HICAL TRANSACTIONS. [ANNO 1J83. 



only, into a paste, it dried very slowly, without producing any perceptible smoke 

 or heat. The quantity perhaps was too small for ignition, and it was probably 

 over-dosed with oil. — Exper. 3. When the mineral was previously calcined with 

 a slight red heat about half an hour, the mixture of it with the oil dried much 

 sooner and harder ; a circumstance which, if not already known, may render it 

 still more valuable to the painter. In other respects no difference could be 

 observed. 



Exper. 4. In the above low heat it suffers no alteration of colour or texture. 

 In a heat of 30°, by the thermometer for measuring high degrees of heat, it 

 loses its property of staining the hands, diminishes very considerably in bulk, 

 acquires a little hardness, though it still proves friable between the fingers, and 

 has its colour changed from a brownish to a blueish black. In a heat of 80° it 

 begins to melt; and at Qb° runs into a black scoria. — Exper. 5. With black 

 flux, in a heat of C)0 , by the above-mentioned thermometer, it yielded a button 

 of lead, amounting, in one experiment to 2 1 , and in another to 22 grains, from 

 an ounce, or nearly ^. 



Exper. 6, Water extracts nothing from it. The mineral acids, with the as- 

 sistance of heat, dissolve about 1 1 parts out of 12 ; but a large quantity of acid 

 is necessary for this solution. The residuum is greyish white, full of bright mi- 

 caceous particles, with a few fine filaments like those of asbestos, which suffer no 

 change in a moderate red heat. In a heat of 141°, which is 14° beyond the 

 fusion of cast iron, it ran into a perfect glass ; but whether this was a vitrifica- 

 tion of the pure earth itself, or of a combination of it with the argillaceous 

 matter it was in contact with, the smallness of the quantity did not admit of as- 

 certaining. On the Hessian crucible it formed a black glass ; what adhered to 

 the thermometer-piece was brown. 



Exper. 7. On boiling with oil of vitriol to dryness, the bottom and sides of 

 the mass became red like colcothar, the middle white, the intermediate parts 

 yellow or reddish yellow, and some greenish. These appearances were at first 

 attributed to a vitriol of iron in different degrees of calcination ; but, on sepa- 

 rating some of the purer white and red parts, the former were found to produce 

 in vitrification the same colour as manganese does, the latter the same as col- 

 cothar ; the other seemed to be a mixture of the two. 



Exper. 8. A solution of the mineral in nitrous acid was precipitated, instead 



common alkali, with Prussian lixivium, which has the property of throwing 

 down from acids, iron, manganese, and all metallic bodies, but no one of the 

 earthy class. When the addition of this lixivium ceased to make any further 

 precipitation, common alkali, added afterwards, had also no effect ; a proof that 

 this mineral contains no earth soluble in acids, for that would have remained in 



