Page:The American Cyclopædia (1879) Volume XI.djvu/727

 MOLECULE T09 until modern times. Now, however, it is one of the most pregnant theories of science ; and it is evident that such motion, if it exists, must be a most important factor in nature. The circumstance that these molecular motions are limited by the boundaries of the mass of mat- ter to which the molecules belong, and that the system remains in equilibrium with rela- tion to external objects, because the amount of motion in opposite directions is usually equal, must not of course affect our estimate of the moving power, and this power is no less than that which would be shown in a motion of translation of the same mass with a velo- city equal to the mean velocity of the several molecules; and, since the facts compel us to assign to this velocity a value commensurable with the velocities obtained in artillery prac- tice, it is evident that the total moving power, even in a small mass of matter, must be enor- mous. There are conditions, however, under which the molecules may communicate their motion to masses and thus produce mechan- ical effects ; and our theory refers the tension of aeriform matter, and the mechanical work which it may be made to do, to the bom- bardment of the sides of the containing ves- sel by molecular projectiles. In a solid or a liquid it is assumed that the extent of the motion of the molecules is limited by internal forces, but in a gas this motion is supposed to be unrestrained, so that the molecules beat freely against any surface with which the aeri- form mass may be in contact, and thus the molecules of water in the cylinder of a steam engine produce their well known effects. The molecular theory has established on a firm foundation the great physical doctrine of the conservation of energy, by explaining a class of phenomena w T hich, as viewed by the old physicists, were apparently wholly at variance with this truth. When two elastic billiard balls strike each other, although the balls may change their velocities, the total moving power will be nearly the same after the collision as before it; but when two inelastic balls of lead strike, there is always an apparent destruction of motion. It was no answer to say that the power which had disappeared as motion had done its work in changing the shape of the balls ; for since these bodies can- not recover their figure, and therefore have not the potential energy of elastic bodies under the same conditions, there must be an annihi- lation of power if the external phenomena are the only effects produced. But if, as our theory assumes, the motion is simply trans- ferred from molar to molecular masses, all is clear ; and since we have been able to prove that the change of temperature produced in the masses is the exact mechanical equivalent of the motion lost, we think we are justified in concluding that the effects ascribed to what we call heat are simply manifestations of mo- lecular motion. When we come to conceive of matter as consisting of elastic molecules which are ever in motion and colliding with each other, we see that motion must be readily communicated from one part of such a system to another ; that any excess of energy acquired by any part must be rapidly dissipated ; and that the tendency must be to bring all the molecules to the same condition. Moreover, we see that the motion must spread not only through the molecules of the same body, but also from one body to another ; for everywhere in nature the atmosphere or some other me- dium furnishes lines of molecules along which the energy can pass. Now exactly this is true of heat. When a heated body is brought into a room, the heat immediately begins to spread through surrounding objects, and the process goes on until all are reduced to what we call a uniform temperature, that is, to a condition in which there is no tendency of heat to pass from one to the other ; and we must remember that our knowledge of temperature and our means of measuring it depend wholly on this motion of heat. We say that one body has a higher temperature or is hotter than another, if when brought in contact heat passes from the first to the second ; and we measure the temperature of a body by bringing in contact with it a thermometer, a small bulb filled with mercury, whose narrow neck enables us to detect the slightest change in the volume of the enclosed liquid. . As this volume increases when the mercury is heated, and diminishes when it is cooled, a fixed position of the mer- cury column indicates that the thermometer is in equilibrium with the body to be tested, and then the artificial scale enables us to compare its thermal condition with freezing and boiling water. Consider next what must be the me- chanical condition of the molecules of two bodies at the same temperature, that is, in thermal equilibrium. The .molecular theory assumes that all the molecules of the same substance are alike in every respect, and there- fore have the same weight ; and hence, in con- sidering the mutual action between different portions of the same substance, we have to deal solely with the collision of small elastic masses of equal weight. Now it follows from the well known laws which govern the collision of elastic bodies, that by the exchanges of velo- city which follow each collision the different portions which we are considering would soon be reduced to a state in which the mean velo- city of the molecules in each part must be equal. Of course the mutual interchange of velocities must continue after the equilibrium is estab- lished, but the loss and gain on either side are then exactly balanced. It follows from this that when two portions of the same sub- stance are in thermal equilibrium, that is, at the same temperature, the molecules of each portion have the same mean velocity. It will be seen however that, although the molecules of a substance in a state of thermal equilibrium have a certain constant mean velocity, the velocity of the individual molecules may vary