^{page 387 note *} The oak-wood lost by thorough drying, at a temperature a little below the scorching point (including a small loss from the action of cold water), 18.3 per cent.; the deal, 14.2; and the pith of the elder, 13.3.

^{page 387 note †} According to Count Rumford (Nicholson's Journal, vol. xxxiv., p. 322), the specific gravity of oak is 15,344; of fir, 14,621, to water as 10,000. He arrived at these results, not by means of the air-pump, but by the expulsion of air by boiling in water. The specific gravity of deal or fir-wood, as given by him, is nearer the truth than that in the text, which is too low, for a reason which will afterwards be assigned. I find, that when air is entirely, or nearly entirely expressed from it by compression in water, that it sinks in a fluid of specific gravity 1.5; and that the pith of the elder, similarly treated, sinks in the same fluid.

The number 1.45, given in the text, as the specific gravity of the pith of the elder, was determined hydrostatically, using a very delicate balance, affected by the one thousandth of a grain, when loaded with 500.

^{page 388 note *} Reduced to powder, after having been subjected to the air-pump, and weighed hydrostatically, it was found to be of the specific gravity 2.41, which is about that of obsidian,—the mineral substance from which, it would appear, that pumice is formed by the action of volcanic fire. As no air was disengaged when the pumice was crushed under water, it seems probable, from the circumstance of its specific gravity being increased by its cells having been broken, that some of them may be destitute even of air. This brings to my recollection the result of an experiment made many years ago, on exposing obsidian to a high temperature in a gun-barrel, in which I assisted a distinguished member of this Society, Sir George Mackenzie. The air disengaged from the obsidian had a distinct smell of nitrous acid gas. Now, supposing that this acid is always set free in the production of pumice from obsidian, part of it may be re-absorbed, and tend perhaps, with steam, to form the minute vacua which I have supposed may exist in pumice,—vacua, the existence of which it is easy to imagine, considering the nature of the substance, in reality a vesicular glass, and differing chiefly from obsidian, or, as it has been significantly called, volcanic glass, in its vesicular condition. This is well displayed by the microscope, under which, with a high power, its minute fragments appear as transparent glass, in some of which cavities are distinguishable.

^{page 388 note †} Henry's Chemistry, vol. ii., p. 506.

^{page 388 note ‡} Whether cork kept in water unaided by pressure, would ever sink, seems very doubtful; probably it would continue to float so long as the plates constituting its cells retained their integrity and elasticity,—that is, so long as its substance resisted decomposition. The portion of cork, the subject of the experiment described in the text, which weighed in air 12.4 grains, after having been in water, under the exhausted receiver 33 days, had increased in weight, from the absorption of the water, to 20.5 grains; and after 22 days more, its farther increase was only 1 grain.

^{page 389 note *} I have found the specific gravity of cambric carefully freed from air 1.600; of hemp cord, 1.560; of fine cotton cloth, 1.605; and of cotton thread, 1.61, at 50° F. The cambric and cotton thread were first boiled in distilled water, and then subjected to the air-pump before weighing in water; they were thoroughly dried before being weighed in air, and weighed whilst still warm. The cord and thread were treated in the same manner, excepting that they were not previously boiled.

^{page 392 note *} Noveau Système, de Chimie Organique, par F. V. Raspail, iii. 406.

^{page 392 note †} Henry's Chemistry, 6th edit. ii. 505.

^{page 393 note *} Since the above was written, I find that Dr J. Lawrence Smith, in a short article on Cholesterine, published in Silliman's Journal for January 1843, has pointed out the common error relative to the specific gravity of this substance, but without endeavouring to determine it exactly. His conclusion was drawn from finding it sink when fused and thrown into water.

^{page 394 note *} Oleine is a frequent accompaniment of cholesterine in animal concretions, and when present, being considerably lighter than water, may be looked for in the film alluded to in the text. Mixed with cholesterine and air, its appearance is very like that of cream on milk.

^{page 394 note †} Researches, Physiological and Anatomical, vol. ii. p. 221. I find that, when milk fresh from the cow is subjected to the air-pump, a small portion of cream soon collects at the surface; and farther, that it may be kept many days (I have kept it twelve days) without any sensible increase in the quantity of cream, or distinct diminution of the opaque whiteness of the milk,—seeming to indicate, that a part of the cream globules, those which rise, may be without an albuminous envelope, and that another part, those which do not ascend, may be provided with such a membrane.

^{page 395 note *} Histoire des Tissus et de la Composition Chimique du Corps Humain, par J. Henle, p. 165.

^{page 395 note †} Butter, I find, when as pure as it can be rendered by melting, is, at the boiling point of water, of the sp. gr. .902; at.100°, apparently .913; and at 48°, .932,—employing the method used in the instances of wax and spermaceti. The lightness of the substance of butter increasing with its temperature, must necessarily expedite the raising of cream, as when the “scalding” process is employed.