That heat is simply atomic motion, and altogether mechanical, is a doctrine which ought never to have been questioned. The interest excited by the bold experiments of Ericson, has caused the scientific to _suspect_, that heat can be converted into motion, and motion into heat--a fact which the author has considered too palpable to deny for the last twenty years. He has ever regarded matter and motion as the two great principles of nature, ever inseparable, yet variously combined; and that without these two elements, we could have no conception of anything existing. It may be thought by some, who are afraid to follow truth up the rugged precipices of the hill of knowledge, that this theory of an interplanetary plenum leads to materialism; forgetting, that He who made the world, formed it of matter, and pronounced it "very good." We may consider ethereal matter, in one sense, _purer_ than planetary matter, because unaffected by chemical laws. Whether still purer matter exists, it is not for us to aver or deny. The Scriptures teach us that "there is a natural body and there is a spiritual body." Beyond this we know nothing. We, however, believe that the _invisible_ world of matter, can only be comprehended by the indications of that which is visible; yet while humbly endeavoring to connect by one common tie, the various phenomena of matter and motion, we protest against those doctrines which teach the eternal duration of the present order of things, as being incompatible with the analogies of the past, as well as with the revelations of the future. FOOTNOTES: [35] Silliman's Journal, vol xxxv., page 283. [36] The real diameter of the earth in that latitude, whose sine is one-third, is a little greater than this; but the true mean is more favorable for the Newtonian law. [37] This is, perhaps, the nearest ratio of the densities and distances. [38] This is an important consideration, as bearing on the geology of the earth. [39] It is not as likely that the condensation of the sun was so sudden as that of the planets, and therefore in this case this distance is only approximate. [40] Mechanique Celeste. Theory of the Moon. [41] Mechanique Celeste. Masses of the planets. SECTION FIFTH. COMETARY PHENOMENA.