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It is proper, however, to observe here, that the various theorems of mechanical science do not rest upon any hypothesis concerning these atoms as a basis. These theorems are not inferred from this or any other supposition, and therefore their truth would not be in anywise disturbed, even though it should be established that matter is physically divisible in infinitum. The basis of mechanical science is observed facts, and, since the reasoning is demonstrative, the conclusions have the same degree of certainty as the facts from which they are deduced.

(19.) Porosity.—The volume of a body is the quantity of space included within its external surfaces. The mass of a body, is the collection of atoms or material particles of which it consists. Two atoms or particles are said to be in contact, when they have approached each other until arrested by their mutual impenetrability. If the component particles of a body were in contact, the volume would be completely occupied by the mass. But this is not the case. We shall presently prove, that the component particles of no known substance are in absolute contact. Hence it follows that the volume consists partly of material particles, and partly of interstitial spaces, which spaces are either absolutely void and empty, or filled by some substance of a different kind from the body in question. These interstitial spaces are called pores.