470 BRITISH SCIENTIFIC ASSOCIATION. 



la the uuiverse may be indefinitely augmented. On the other hand, the identifica* 

 lion of radiant heat with light, and the establishment 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, indicate a mechanism of a more complicated 

 kind than that of light, and leave ample room for further speculation. The only 

 apeehank-al hypothesis (so far as I am aware) which is consistent with the present 

 state of our knowledge of the phenomena of heat, is the theory of molecular vor- 

 tices of Mr. Rankine, In this theory all bodies are supposed to consist of atom&, 

 composed of nuclei surrouuded with elastic atmospheres. The radiation of light 

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

 mometric heat is supposed to consist in circulating currents or vortices, amongst 

 €be 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-dynamica, by the application of known mechanical prin- 

 ciples. He has also, from the same principles, deduce! relations (which have 

 been confirmed by experiment) between the pressure, density and absolute tem* 

 perature of elastic fluids, and between the pressure and temperature of ebullition 

 of fluids. The dynamical theory of heat enables us to frame some conjectures to 

 account for the continuance of its supply, and even to speculate as to its source. 

 The heat of the sun is dissipated and lost by radiation, and must be progressively 

 diminished, unless its thermal energy be supplied. According to the measurements 

 of M. Pouillet, 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 seventeen 

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

 water, and the heat be supposed to be drawn uniformly from its entire mass, its 

 temperature would thereby undergo a diminution of 2 ° .4 Fahr. annually. On 

 the other hand, there is a vast store of force in our system- capable of conversion 

 into heat. If, as is indicated by the small density of the sun, and by other circum- 

 stances, that body has not yet reached the condition of incompressibility, we have, 

 in the future approximation of its parts, a fund of heat probably quite large 

 enough to supply the wants of the human family to the end of its sojourn here. 

 It has been calculated that an amount of condensation, which would diminish the 

 diameter of the sun by only the ten-thousandth part, would suffice to restore the 

 heat emitted in 2,000 years. Again, on our own earth, vis viva is destroyed by 

 friction in the ebb and flow of every tide, and must therefore reappear as heat 

 The amount of this must be considerable, and should not be overlooked in any 

 estimation of the physical changes of our globe. According to the computation 

 of Bessel, 25,000 cubic miles of water flow in every six hours from one quarter of 

 the earth to another. The store of mechanical force is thus diminished and the 

 temperature of our globe augmented by every tide. We do not possess the data 

 which would enable us to calculate thcmagnitude of these effects. All that we 

 know with certainty is, that the resultant effect of all the thermal agencies to 

 which the earth is exposed has undergone no perceptible change within the his- 

 toric period. We owe this fine deduction to Arago. In order that the date palm 

 should ripen its fruit, the mean temperature of the place must exceed TO ° Fahr.; 

 and, on the other hand, the vine cannot be cultivated successfully when the tem- 

 perature is 12° or upwards. Hence, the mean temperature of any place at 

 which these two plants flourished and bore fruit must lie between these narrow 



