94 Prof. Challis on the Theory of 



tion appears to me to depend on a property of elastic fluids rela- 

 tive to heat which has been experimentally ascertained by Prof. 

 Tyndall. The experiments show that certain fluids, as atmo- 

 spheric air and simple gases, are, like rock-salt, permeable to 

 heat-undulations, just as transparent substances are permeable 

 to light-undulations. Consequently in these fluids the addition 

 or abstraction of heat-undulations is not accompanied by such 

 disturbance, or breaking up, of the undulations as would pro- 

 duce change of temperature and radiation. Now if we may 

 suppose that the heat-undulations from the atoms of these same 

 fluids always remain in like manner undisturbed when the atoms 

 are caused to approach each other by compression, or to recede 

 from each other by dilatation, we shall have reason for conclu- 

 ding that the sudden changes of density in vibrations of the 

 fluid do not give rise to changes of temperature and elasticity, 

 but only produce the changes of caloric repulsion to which are 

 due the changes of pressure. By this process of reasoning we 

 are evidently conducted to the inference that the actual velocity 

 of sound should be mathematically deducible, at least very ap- 

 proximately, on the supposition that the elasticity of the fluid is 

 the same in a state of vibration as when it is at rest. I have, 

 in fact, obtained in this way a velocity very little less than that 

 given by direct observation. 



It is, however, an ascertained fact that sudden condensations 

 of the air develope heat, and sudden rarefactions abstract heat. 

 A physical explanation of these effects may be given on the follow- 

 ing principles. "When there is an equilibrium of temperature, as 

 much heat is received within a given space as issues from it, 

 whatever be the density of the substances in the space. Hence 

 as a sudden condensation, by bringing into a given space an 

 additional number of atoms, suddenly increases the number of 

 centres of heat-undulations of given specific intensity, the equi- 

 librium of temperature is momentarily destroyed. A like effect 

 is produced by a sudden rarefaction. But while such effects are 

 being produced, the dispersion, by radiation, of the excess or 

 defect of heat to surrounding substances, is continually tending 

 to restore the equilibrium, When it is considered that this 

 radiation traverses space much more rapidly than the vibrations 

 of the elastic medium, and that consequently simultaneous effects 

 of the excess and the defect of heat may at any given point very 

 nearly neutralize each other, and also that time is always required 

 to produce changes of temperature, there seems no good reason 

 to conclude that any but inconsiderable changes of temperature 

 and elasticity result from this alternate generation and absorp- 

 tion of heat. The changes, for instance, in unconfined air can- 

 not be comparable with those which would be caused by the 



