Page:Popular Science Monthly Volume 10.djvu/724

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find only that the flow of the currents—the motion of the masses—is proportionately increased. Is there a connection of cause and effect between these phenomena?

All motion that we are familiar with requires the expenditure of heat. The combustion of coal supplies motion to the steam-engine. The evaporation of water by the sun's heat causes the rain-clouds and the mill-streams. The oxidation of certain elements in the food we eat is the combustion which supplies our bodies with powers of motion. Recent discoveries have shown not only that motion is heat transformed, but that to produce a certain quantity of motion an invariable certain quantity of heat is required.

Again, the cessation of motion evolves heat. It is well known that by skillful blows with the hammer a cold iron bar can be made red-hot. Two wheels revolving in opposite directions, and touching at the circumference, become highly heated; and factories have been warmed solely by this transfer of motion into heat. Friction is but another name for the arresting of motion, and, as we well know, always produces heat. There is also here the same equivalence as in the other case. The stoppage of motion evolves just the amount of heat that was required to produce that motion.

The greatest triumph of modern science is the splendid induction that all the forces are correlative and indestructible. Not an impulse of motion, of light or heat, or any force, is ever lost. It may be communicated from one body to another, or transmuted into some other form of force, or become for a time latent or imperceptible; but it always exists, and is reclaimable back again into the same, in mode and quantity, from which it started.

The grandest exemplification of these truths will be found in what we are now considering, the origin of the celestial revolutions. The condensation of gases gives out heat in direct proportion to the contraction of volume. The attraction of gravitation, not only between masses but between all the particles of matter, increases in the inverse square of the diminishing distance. From these two principles it can be mathematically shown that in the contraction of each great world-nebula heat would be set free in the precise proportion of the increase of atomic attraction; or, in other words, that it would take the exact amount of heat-force that had been released, to separate the atoms again to their original distance apart. But in this instance the heat-force is not really set free; it is transformed into the motion of the mass from which it came. Instead of holding the atoms apart, the work which it now has to do under the form of motion is, to prevent the masses from falling into each other. It is this motion—the celestial revolutions—which keeps the worlds apart, and allows each to work out its destiny under the aggregating forces, without interference from any other. Up to a certain point of condensation, which is previous to the radiation of heat into space, if this motion