VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 28p 



crystallization was formed : the dry salt appeared merely alkaline as at first, and 

 deliquiated in the air; a further proof that this clay contains no gypseous 

 matter; for the vitriolic acid would have been absorbed by the alkali, and formed 

 vitriolated tartar, a salt which neither liquefies in the air, nor dissolves easily in 

 water, and which therefore would have crystallized long before the alkali became 

 dry, or remained after its deliquiation. A 20th part of gypsum, ground with 

 clay, was very distinguishable by both the foregoing processes; producing a sul- 

 phureous smell; and calcareous earth by calcination with charcoal powder; and 

 crystals of vitriolated tartar by calcination with the same alkaline salt. 



To separate the pure argillaceous part, or that matter which in all clays forms 

 alum with the vitriolic acid, 240 grains of this clay were thoroughly moistened 

 with oil of vitriol, boiled to dryness, and at last made nearly red-hot. The mix- 

 ture was then boiled in water; the earth which remained undissolved was treated 

 again in the same manner with vitriolic acid, and this operation repeated 5 or 6 

 times. The clay was diminished in the first operation about 70 grains; but less 

 and less in the succeeding ones, and in the last scarcely 2 grains. The filtered 

 liquors yielded crystals of true alum; but its quantity was not examined, as the 

 produce of alum from aluminous earth is already sufficiently known, and the 

 quantity of aluminous earth itself, or its proportion to the indissoluble earth, 

 was here the object. From the 240 grains of clay there remained in one expe- 

 riment 98, and in another 95 grains of indissoluble earth; so that 5 parts of 

 this clay consist of 3 parts of pure argillaceous or alum earth, and 2 parts of an 

 earth of a different kind. 



With respect to the nature of this last earth, it is easier to determine nega- 

 tively what it is not, than positively what it is; but ascertaining the former will 

 be a great step towards the discovery of the latter. That it is not calcareous, 

 gypseous, or argillaceous, is manifest from the experiments. — It is not jasper; 

 as this consists, in great part, of argillaceous earth, which would have been 

 extracted by the vitriolic acid. — It is not fluof; as this, by the same acid, would 

 have been* decomposed, its own acid expelled, and a gypseous earth left. — It is 

 not of the micaceous kind; as the peculiar aspect of these earths would readily 

 betray them to the eye. — It is not granite; for strong fire, which granite melts 

 in, has no effect on this. Nor is there any known kind of earth to which it is 

 in any degree similar, except those of the siliceous order; and with these it per- 

 fectly agrees in all the properties I am acquainted with, that they possess in a 

 state of powder. 



It does not vitrify or soften with pure clay, in the strongest fire I have been 

 able to produce. Nor is it disposed to melt with the matter of Hessian cruci- 

 bles; for a little of it rubbed on the inside of a crucible, and urged with strong 



vol. xv. P p 



