162 ANNUAL OF SCIENTIFIC DISCOVERT. 



The mechanical theory of heat rests upon a wide basis, and proofs in 

 verification of tin- theory arc constantly accumulating. When the weight of 

 any liquid whatever is known, with the comparative weight of its vapor at 

 different pressures, the latent heat at the different pressures is readily esti- 

 mated from the theory; and this method of estimation agrees with the best 

 experimental results, as may afterwards be shown; and when the latent heat 

 i~ also known, the specific heat of the liquid can be determined by means of 

 the same theory; in other words, the quantity of work, in foot-pounds, may 

 be determined, which would, by agitating the liquid or by friction, be required 

 to raise the temperature of any given quantity of the liquid by, say, one 

 degree, altogether independently of Joule's experiments. The theory enables 

 us to discover the utmost power it is possible to realize from the combination 

 of any given weight of carbon and oxgyen, or other elementary substances, 

 with nearly as much precision as we can estimate the utmost quantity of 

 work it is possible to obtain from a known weight of water falling through 

 a given height. It is not difficult to comprehend, then, that the theory of 

 the mechanical equivalent of heat proves of great practical utility. 



According to the mechanical theory of heat, in its general form, heat, 

 mechanical force, electricity, chemical affinity, light, sound, are but different 

 manifestations of motion. Dulong and Gay-Lussac proved by their experi- 

 ments on sound that the greater the specific heat of a gas, the more rapid 

 are its atomic vibrations. Elevation of temperature does not alter the 

 rapidity, but increases the length of the vibrations, and, in consequence, 

 produces "expansion" of the body. All gases and vapors are assumed to 

 consist of numerous small atoms, moving or vibrating in all directions with 

 great rapidity; but the average velocity of these vibrations can be estimated 

 when the pressure and weight of any given volume of gas is known, pressure 

 being, as explained by Joule, the impact of those numerous small atoms, 

 striking in all directions, and against the sides of the vessel containing the 

 gas. The greater the number of these atoms, or the greater their aggregate 

 weight, in a given space, and the higher the velocity, the greater is the pres- 

 sure. A double weight of a perfect gas, when confined in the same space, 

 and vibrating with the same velocity, that is, having the same tempera- 

 ture, gives a double pressure; but the same weight of gas, confined in the 

 same space, will, when the atoms vibrate with a double velocity, give a 

 quadruple pressure. An increase or decrease of temperature is simply an 

 increase or decrease of molecular motion. The truth of this hypothesis is 

 very well established, as already intimated, by the numerous experimental 

 facts with which it is in harmony. 



V* lien a gas is confined in a cylinder under a piston, so long as no motion 



is given to the piston, the atoms, in striking, will rebound from the piston 



after impaet with the same velocity with which they approached it, and no 



motion will be lost by the atoms. But when the piston yields to the pressure, 



the atoms will not rebound from it with the same velocity with which they 



strike, but will return after each succeeding blow with a velocity continually 



decreasing as the piston continues to recede, and the length of the vibrations 



be diminished. The motion gained by the piston will, it is obvious, be 



isely equivalent to the energy, heat, or molecular motion, lost by the 



Vibratory motion, or heat, being converted into its equivalent 



of onward motion, or dynamical effect, the conversion of heat into power, or 



of power into heat, is thus simply a transference of motion; and it would 



be as reasonable to expect one billiard-ball to strike and give motion to an- 



