The author enters on the subject by adverting to the apparent lightness imparted to the common woods, and to certain vesicular minerals, by the entanglement of air in their substance, as is commonly understood, and as is proved by the action of the air-pump. The specific gravity of oak-wood, after having been kept under the exhausted receiver till it sunk in water, and ceased to give off air, he found to be (inclusive of hygrometric moisture) 1.58; that of deal, 1.18, or when crushed about 1.5; that of the pith of the elder, 1.45; and that of pumice, 1.94, or crushed, 2.41, which is nearly the same as that of obsidian, from which pumice appears to be formed by the action of volcanic fire.