VOL. LXXXr.] PHILOSOPHICAL TRANSACTIONS. ' lOQ 



A bit exposed on the charcoal to the blow-pipe became black, melted like some 

 vegetable matters, caught flame, and burnt to a botryoid inflated coal, which 

 soon entirely consumed away, and vanished. A piece put into water fell to a 

 powder. The mixture being boiled, this powder dissolved, and turned the whole 

 to a jelly. These properties are exactly those of common starch. 



N° 5, agreed entirely with N° 4, in appearance, properties, and nature. 



N° 6. The pieces of this parcel were white, quite opaque, and considerably 

 hard. Their taste and effects at the blow-pipe, were perfectly similar to those 

 of the Hydrabad kind. 



N° 7 much resembled N° 6, only was rather softer, and seemed to blacken a 

 little when first heated. With fluxes at the blow-pipe it showed the same effects 

 as all the above. 



Conclusion. — 1. It appears from these experiments, that all the parcels, except 

 N°4and5, consisted of genuine tabasheer ; but that those kinds immediately 

 taken from the plant, contained a certain portion of a vegetable matter, which 

 was wanting in the specimens procured from the shops, and which had probably 

 been deprived of this admixture by calcination, of which operation a partial black- 

 ness, observable on some of the pieces of N° 3 and 6, are doubtless the traces. 

 This accounts also for the superior hardness and diminished tastes of these sorts. 



2. The nature of this substance is very difi^erent from what might have been 

 expected in the product of a vegetable. Its indestructibility by fire ; its total re- 

 sistance to acids ; its uniting by fusion with alkalis in certain proportions into a 

 white opaque mass, in others into a transparent permanent glass ; and its being 

 again separable from these compounds, entirely unchanged by acids, &c. seem to 

 afford the strongest reasons to consider it as perfectly identical with common 

 siliceous earth. Yet from pure quartz it may be thought to differ in some mate- 

 rial particulars; such as in its fusing with calcareous earth, in some of its effects 

 with liquid alkalis, in its taste, and its specific gravity. But its taste may arise 

 merely from its divided state, for chalk and powdery magnesia both have tastes, 

 and tastes which are very similar to that of pure tabasheer; but when these earths 

 are taken in the denser state of crystals, they are found to be quite insipid ; so 

 tabasheer, when made more solid by exposure to a pretty strong heat, is no 

 longer perceived, when chewed, to act on the palate, § 4 (a). 



And, on accurate comparison, its effects with liquid alkalis have not appeared 

 peculiar; for though it was found on trial, that the powder of common flints, 

 when boiled in some of the same liquid caustic alkalis employed at ^ g (a), was 

 scarcely at all acted on : and that the very little which was dissolved, was soon 

 precipitated again, in the form of minute flocculi, on exposing the solution to the 

 air, and was immediately thrown down on the admixture of an acid ; yet the pre- 

 cipitate obtained from liquor silicum by marine acid was discovered, even when 



