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On the other hand, Newton, perhaps the most exact thinker known to scientific history, has expressed opinions on the constitution of matter closely resembling those of Lucretius. The latter asserts that all substance consists of atoms, which are perfectly solid, and therefore incapable of being crushed or torn apart; for that which has no void within itself can not be separated into parts: moreover, they are exceedingly hard, for otherwise they could not form hard bodies like iron; yet, when combined with "much void," they can give rise to soft substances, as water and air. Compare this statement with Sir Isaac Newton's belief, as expressed in the following terms: "It seems probable that God, in the beginning, formed matter in solid, massy, hard, impenetrable, movable particles of such size, figures, and with such other properties, and in such proportion to space, as most conduced to the end for which he formed them; and that these primitive particles, being solid, are incomparably harder than any porous body compounded of them—even so very hard as never to wear, or to break in pieces." If we except the belief in the creative power of God, this quotation gives us Lucretius's atomic theory in a nutshell.

Our author is again in harmony with the latest deductions of physics, when he asserts that the atoms have in themselves no sensible properties, such as color, heat, etc. But the arguments which he uses to establish this proposition are by no means convincing. His treatment of the atomic motions, however, is the most vulnerable point in his system. He supposed all constituted things to be produced by the impact of atoms, which through all eternity were descending, urged on by their own weight. Now, Lucretius had very clear ideas on the subject of gravitation. He knew that, except when in a resisting medium, all bodies fall with equal velocities. Hence, in this everlasting, downward rain, it would be impossible for one atom to approach another and combine with it. To obviate this difficulty, he conceived a slight lateral motion, by which the particles are brought together. He offers no reason for this extraordinary hypothesis, except that no other supposition can explain the formation of things so as to accord with his theory. It is the old story of system first and facts afterward, and shows well the injurious tendency of the a priori method in one who was otherwise well fitted for the pursuit of knowledge.

Passing on to his other teachings, we find him devoting a whole book to the bodily sensations. These, he attempts to show, are produced by corporeal images given off from bodies, and coming into contact with our organs of sense. Thus, he thought that all things were giving off thin pellicles of substance, which, by impinging upon the eye, cause the sensation of sight. This is not unlike Newton's emission hypothesis. His theory of sound also is more purely mechanical than that at present accepted. He supposes this sensation to be caused by the direct passage of particles from the source of sound to the