1Q8 PHILOSOPHICAL TRANSACTIONS. [ANNO 1798. 



In a subsequent experiment on the same, the crucible was removed immediately 

 after the last projection, and I then observed that an effervescence, with a disen- 

 gagement of carbonic acid, took place, on the addition of the muriatic acid, as is 

 usual when pure plumbago is decomposed by nitre, and that less of the gelatinous 

 matter was formed by evaporation. The cause of this difference was evidently the 

 duration of the red heat ; for in the first instance the alkali developed by the de- 

 composition of the nitre had time to unite with the siliceous earth, so as, when dis- 

 solved, to form liquor silicum ; but in the 2d ex- 



* [ Grains. 



periment a portion of alkali remained combined ^ r d. 0.30 



with the carbonic acid, produced by the carbon of Alurmna "' p^o 



the decomposed plumbago. The produce of 100 Oxide of iron e. 3.20 



grs. by this analysis was as annexed. Mr. Wedg- Graphite or plumbago ... u 10.25 



wood says, that sulphuric acid cannot dissolve the 



precipitated earth, and has but little effect on the ^8.40 



mixed substance, even when distilled to dryness ; but, from the preceding experi- 

 ments, I had reason to believe that the aluminous earth and iron would be sepa- 

 rated by reiterated distillation ; I therefore repeated the analysis in the following 

 manner. 



Second analysis of the Sydneia, N° 2. — A. 100 grs. of the earth were put into a 

 glass retort, on which 400 grs. of pure concentrated sulphuric acid were poured. 

 The retort was placed in a small reverberatory, and the fire was continued till a dry 

 mass remained. 400 grs. of the acid were again poured in, and distilled as before. 

 On the dry mass, boiling water was poured, and the whole was then emptied on a 

 filter, and edulcorated. The residuum, after a red heat, weighed 87*75 grs., and 

 consisted of siliceous earth, mixed with some mica, and with particles of plumbago. 



b. The filtrated solution, by ammonia, afforded a precipitate, which weighed 

 g.50 grs. ; and, being examined, as in the former experiment, yielded 6.50 grs. of 

 alumina, and 3 errs, of oxide of iron. The plum- . . 



1 1 •!• • Grains. 



bago was separated from the siliceous matter, in Silica and mica 77. .75 



the manner already described, and amounted to ^ mm ^*. „ ° 



* \ t Oxide or iron ., . . 3 



about 1 grs. By this analysis I obtained as an- Plumbago 10 



nexed: it appears therefore that the Sydneian earth, "07T5 



when treated with sulphuric acid, is capable of 



being for the greater part decomposed ; and Mr. Wedgwood probably did not suc- 

 ceed, because his process was in some respect different, or that the distillation was 

 not sufficiently repeated. I have not thought it necessary to be more circumstantial 

 in the account of this 2d analysis, as the operations were similar to those of the 

 former. 



§ 4. These experiments prove, that the earthy substance called Sydneia or terra 

 australis, consists of siliceous earth, alumina, oxide of iron, and black lead or 

 graphite. The presence of the latter appears to be accidental, and it probably was 



