But the effects which visible oscillations and oscillations sensible to touch, have in facilitating the re-arrangement of parts by an incident force, are insignificant compared with the effects which insensible oscillations have in aiding such change of structure. It is a doctrine now generally accepted among men of science, that the particles of tangible matter, as well as the particles of ether, undulate. As interpreted in conformity with this doctrine, the heat of a body is simply its state of molecular motion. A mass which feels cold, is one having but slight molecular motion, and conveying but slight molecular motion to the surrounding medium or to the hand touching it. A mass hot enough to radiate a sensible warmth, is one of which the more violently agitated molecules, communicate increased undulations to the surrounding ethereal medium; while the burn inflicted by it on the skin, is the expression of increased undulations of the organic molecules. Such further heat as produces softening and a consequent distortion of the mass, is an agitation so much augmented that the units can no longer completely maintain their relative positions. Fusion is an agitation so extreme, that the relative positions of the units are changeable with ease. When, finally, at a still higher temperature, the liquid is transformed into a gas, the explanation is, that the oscillations are so violent as to overbalance that force which held the units in close contiguity—so violent as to keep the units at those relatively great distances apart to which they are now thrown. Since the establishment of the correlation between heat and motion first gave probability to this hypothesis, it has been receiving various confirmations—especially by recent remarkable discoveries respecting the absorption of heat by gases. Prof. Tyndall has proved that the quantity of heat which any gas takes up from rays of heat passing through it, has a distinct relation to the complexity of the atoms composing the gas. The simple gases abstract but little; the gases composed of binary atoms abstract, say in round numbers, a hundred times as much; while the gases composed of atoms severally containing three, four, or more simple ones, abstract something like a thousand times as much. These differences Prof. Tyndall regards as due to the different abilities of the different atoms to take up, in the increase of their own undulations, those undulations of the ethereal medium which constitute heat—an interpretation in perfect accordance with the late results of spectrum-analysis; which go to show that the various elementary atoms, when in an aeriform state, intercept those luminiferous vibrations of the ether which are in unison or harmony with their own. And since it holds of solid as of gaseous matters, that those consisting of simple units transmit heat far more readily than those consisting of complex units; we get confirmation of the inference otherwise reached, that the