Porism was also used by the Greek geometers to denote a corollary to a proposition, and the frequent use of the word in this sense, as well as in the other, by Pappus and Proclus, has occasioned much con fusion. Proclus says that "corollary is one of the geometrical appel lations, but it has a twofold signification," and he proceeds to describe, in a very obscure manner, the difference between the two meanings of the term.
(See Praia* in Euclidem, edit. Hervagii, fol. Basil., 1533, foL 18. We refer the reader also to Henry Savile's Praketioncs in L'adidcm, 4to, Oxon, 1621, p. 18 ; and Trail's Life of Simeon, p. 92.) is that condition of material bodies which consists in the discontinuity of their molecules, the intervals between these being called pores (from wtipos, a passage). Porosity is a property common to all bodies in nature, at least we know none in which the particles are contiguous to one another. By pores however we do not mean the cavities as in sponge and cork, which:are visible to the eye and scarcely those of other bodies which may be rendered so by the aid of a micro scope. In bodies whose pores are not manifest, the existence of the intervals between the molecules is proved by various circumstances. Thus many of the metals become more compact by hammering, and all of them contract in hulk by a reduction of temperature. We may also refer to the Florentine experiment, for determining whether or not water is compressible,—the fluid was by pressure forced through the pores of the vessel of gold in which it was contained. Again, the porosity of bodies is inferred from their elasticity and the sounds which are heard'when the molecules are in a state of vibration : also, in transparent bodies it is inferred from the fact that the particles of light pass through them, or that the vibrations of an tetherial fluid take place among the molecules.
When salt is dissolved in water, the particles of the salt seem to introduce themselves between those of the water, so that the volume of the mixture is less than the sum of the volumes of the separate substances; and the like may he said of the mixture of alcohol with water ; in which eases the particles of one of tho kinds of substance appear to enter and occupy the spaces between the particles of the other. The intervals between the particles of gaseous substances are
very great ; and though, in some eases, the volume of a mixture is equal to the sum of the volumes of the separate gases, yet, in others, it is equal to not inure than 4,1, or A of the sum of the separate volumes. A body of aqueous vapour composed of a volume repro., Nented by 2r of hydrogen gas, and a volume o of oxygen gm, Is equal In volume to 2r only.
All material substances being subject to attractive forces, it has been made a question whether tho attractions which take place between the molecules of bodies, and which aro Insensible at all appreciable distances from them, are the same as that general attraction which extends Indefinitely through space ; modified, however, by the figures and mutual distances of the molecules, by heat, electricity, and perhaps by powers which are at present unknown to us; but in order that this hypothesis may be admissible, the dimensions of the molecules of belies should bo extremely small compared with those of the spaces among them; and the densities of the molecules Immensely greater than the densities of the bodies themselves. La Place estimates (' Systeme du Monde,' ch. xviii. 4th edit) that a molecule of a spherical form, whose diameter is one millionth part of a metre, ought to have a density more than six million times as great as the mean density of the earth in order that it might exercise an attraction equal to that of terrestrial gravity ; and he observes, that the attractive forces exercised by the molecules of bodies, which are probably only the excesses of the entire attractions of the molecules over the repulsive forces of the caloric in the intervals, must be vastly greater than that of gravity, since the actions of the molecules of a body produce visible inflexions of the rays of light, which cannot bo asserted concerning the attraction of gravity. [Arrascriox.]