194 ANNUAL OF SCIENTIFIC DISCOVERY. 



cases in which heat is developed by mechanical action, or vice versa, a defi- 

 nite relation would be found to subsist between the amount of the action 

 and that of the heat developed or absorbed. This conjecture was put to the 

 test of experiment by Mayer and Joule, in 1842, and was verified by the re- 

 sult. It was found that heat and mechanical power were mutually convertible; 

 and that the relation between them was definite., 772 foot-pounds of motive 

 power being equivalent to a unit of heat that is, to the amount of heat re- 

 quisite to raise a pound of water through one degree of Fahrenheit. The 

 science of Thermodynamics, based upon this fact, and upon a few other ob- 

 vious facts or self-evident principles, has grown up in the hands of Clausius, 

 Thomson, and Ilankine, into large proportions, and is each day making fresh 

 conquests from the region of the unknown. Thus far the science of heat 

 is made to rest wholly upon the facts of experiment, and is independent of 

 any hypothesis respecting the molecular constitution of bodies. The dynam- 

 ical theory of heat, however, lias materially aided in establishing true phys- 

 ical conceptions of the nature of heat. The old hypothesis of caloric, as a 

 separate substance, was indeed rendered improbable by the experiments of 

 Rumford and Davy, and by the reasonings of Young ; but it continued to 

 hold its ground, and is interwoven into the language of science. It is now 

 clearly shown to be self-contradictory ; and to lead to the result that the 

 amount of heat in the universe may be indefinitely augmented. On the 

 other hand, the identification of radiant heat with light, and the establish- 

 ment of the wave-theory, left little doubt that heat consisted in a vibratory 

 movement either of the molecules of bodies or of the ether within them. 

 Still, the relation of heat to bodies, and the phenomena of conduction, indi- 

 cate a mechanism of a more complicated kind than that of light, and leave 

 ample room for farther speculation. The only mechanical hypothesis (so 

 far as I am aware) which is consistent with the present state of our knowl- 

 edge of the phenomena of heat, is the theory of molecular vortices of Mr. 

 Rankine. In this theory all bodies are supposed to consist of atoms, com- 

 posed of nuclei surrounded with elastic atmospheres. The radiation of light 

 and heat is ascribed to the transmission of oscillations of the nuclei ; while 

 tJiermometric heat is supposed to consist in circulating currents or vortices, 

 amongst the particles of their atmospheres, whereby they tend to recede from 

 the nuclei, and to occupy a greater space. From this hypothesis Mr. Rankine 

 has deduced all the laws of thermo-dynamics, by the application of known 

 mechanical principles. He has also, from the same principles, deduced rela- 

 tions (which have been confirmed by experiment) between the pressure, den- 

 sity and absolute temperature of elastic fluids, and between the pressure and 

 temperature of ebullition of liquids. The dynamical theory of heat ena- 

 bles us to frame some conjectures to account for the continuance of its sup- 

 ply, and even to speculate as to its source. The heat of the sun is dissi- 

 pated and lost by radiation ; and must be progressively diminished unless 

 its thermal energy be supplied. According to the measurements of M. Pou- 

 illet, the quantity of heat given out by the sun in a year is equal to that 

 which would be produced by the combustion of a stratum of coal seven- 

 teen miles in thickness ; and if the sun's capacity for heat be assumed equal 



