I. MATTER 13 II. PROPERTIES OF MATTER 19 III. THE ESSENTIAL PROPERTIES OF MATTER 33 IV. ATTRACTION 38 V. GRAVITATION 51 VI. CENTRE OF GRAVITY 67 VII. HYDROSTATICS 80 VIII. SPECIFIC GRAVITY 100 IX. PNEUMATICS 110 X. MOTION 133 XI. THE MECHANICAL POWERS 174 XII. SOUND 193 XIII. HEAT 207 XIV. LIGHT 258 XV. ELECTRICITY 287 XVI. MAGNETISM 308 NATURAL PHILOSOPHY. CHAPTER I. MATTER. 1. =Matter and Spirit.=--The distinction between matter and spirit is almost universally recognized even by those who have given little thought to such subjects. It is a distinction of which we are conscious in our own persons. We know instinctively that there is a something within us that causes the movements of our material bodies, and that something we call spirit. 2. =Bishop Berkeley's Ideas.=--Some philosophers, in their speculations, have denied that there is any such thing in existence as matter. Bishop Berkeley, for example, taught that the impressions which we suppose that we receive from material objects do not come from real substances, but are the "effects of the immediate agency of an ever-present Deity." It is no wonder that the wisdom and learning of a man who could seriously adopt such a belief could not save him from being the dupe of quackery. He believed that tar-water was a sovereign cure for all diseases; and Dr. Holmes playfully remarks of him, that "he held two very odd opinions: that tar-water was every thing, and that the whole material universe was nothing." 3. =Hume's Ideas.=--The infidel Hume went beyond Bishop Berkeley, denying even the existence of the soul as an individual and responsible agent. He made every thing to consist of ideas and impressions, and said that these have no necessary connection, but are "a bundle of perceptions that succeed each other with inconceivable rapidity, and that therefore I myself of to-day am no more the I myself of yesterday or to-morrow than I am Nebuchadnezzar or Cleopatra." A wag proposed the following epitaph for his tomb-stone as a suitable illustration of his theory: "Under this circular idea, vulgarly called tomb, Impressions and ideas rest which constituted Hume." 4. =Origin of the word Spirit.=--The name spirit came originally from an attenuated form of matter, the air or breath, because the air, like spiritual existence, is invisible. The formation of language is to a large extent thus based on analogies.