P.S. I have a part of a _rock crystal_, which contains, in a hollow, a _liquid_ and a _little air_, and some _black_ or _brown particles_, which just sink in the liquid. It was found several years since at Diamond Hill in Catskill. This hill is only a small eminence on the bank of the creek at that place, composed of limestone, (if I have been correctly informed,) between the strata of which, and on the side next the creek, this and other rock crystals were found. I believe, Sir, you have one like the above, obtained from the same place. The crystal, which was generously given me by Mr. Van Loon, who found it, is only a part of two crystals connected at their bases. Partly under one of the solid angles formed by the united pyramids, is the hollow, about ⅝ inch long, about ⅜ filled with the air, and about ¼ inch wide. The principal curiosity about it is the liquid. It has never been known to _freeze_. It was exposed yesterday morning an hour to an atmosphere 4 and 5 degrees below zero. It became less fluid, for the bubble of air moved with less ease and rapidity. Still the liquid was fluid. Its colour, which is naturally white, had a slight tinge of yellow. The Rev. Mr. Schaeffer of New-York supposes the black particles are bitumen. Is it possible the liquid is _naptha_? This oil is sometimes colourless, and does not congeal at zero, and that which I distilled from the Seneca oil, does congeal at some degrees below zero. It can hardly be _salt water_, unless it be _very salt_, and even then, it would have congealed at the temperature of the air yesterday. What way can be devised to ascertain what it is? _Jan. 30, 1819._ After seeing the notice of the crystals found at Hudson by Mr. Schaeffer, I wrote to a member of the Lyceum of Natural History, New-York, rather more full an account than the above, of my crystal, &c. I hope to ascertain, whether the liquid will congeal at 10 or 20° below 0, but have some fear lest the crystal should be injured. C. D. ART. IV. _On the Tourmalines and other Minerals found at Chesterfield and Goshen, Massachusetts, by Col._ GEORGE GIBBS. (For the American Journal of Science.)