i7<> Mr. Smithson on the Composition 
J s well known, having mentioned to me, in one of his letters* 
that from some experiments of his own, he was led to sus- 
pect the existence of phosphoric acid in several stones, and 
particularly in the zeolite of Auvergne, I have directed my 
enquiries to this point, but have not found the phosphoric, or 
any other acknowledged mineral acid, in this zeolite. 
Many persons, from experiencing much difficulty in com- 
prehending the combination together of the earths, have been 
led to suppose the existence of undiscovered acids in stony 
crystals. If quartz be itself considered as an acid, to which 
order of bodies its qualities much more nearly assimilate it, 
than to the earths, their composition becomes readily intelli- 
gible. They will then be neutral salts, silicates, either simple 
or compound. Zeolite will be a compound salt, a hydrated 
silicate of alumina and soda, and hence a compound of alumina 
not very dissimilar to alum. And topaz, whose singular in- 
gredients, discovered by Mr. Klaproth, have called forth a 
query from the celebrated Mr. Vauquelin, with regard to 
the mode of their existence together,* will be likewise a com- 
pound salt, consisting of silicate of alumina, and fl uate of 
alumina. 
Our acquaintance with the composition of the several mineral 
substances, is yet far too inaccurate to render it possible to 
point out with any degree of certainty, the one of which zeo- 
lite is an hydrate, however the agreement of the two substances 
in the nature of their constituent parts, and in their being both 
electrical by heat, directs conjecture towards tourmaline. 
St James's Place , Jan . 22, 1 8 1 1 . 
* Annales du Museum d’Hist. Nat. tome 6, p, 24. 
