7 o4 THE POPULAR SCIENCE MONTHLY. 



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 com- 

 municated 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 con- 

 traction 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 princi- 

 ples 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 mo- 

 tion 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 



