346 Mr. James Croll on the Nature of Heat Vibrations. 



the heat really remaining with the gas may be correctly evalu- 

 ated. It would be superfluous to attempt to indicate on this 

 occasion the proper means by which the difficulties just explained 

 might be either eluded or overcome — especially as, in the case 

 of the first-mentioned, suitable expedients could only be disco- 

 vered from actual trials, and would depend on the choice of 

 substance of the globe, whilst with regard to the last-mentioned 

 the means to be employed are sufficiently familiar. Thus much, 

 however, may be stated, that none of the difficulties enumerated 

 (and they seem, at least, all the more important ones) appear 

 greater than what are habitually encountered in researches of 

 this nature. 



Oxford, March 1864. 



LV. On the Nature of Heat Vibrations. 

 By Mr. James Croll *. 



IN a most interesting paper on Radiant Heat, by Professor 

 Tyndall, read before the Royal Society in March last, it is 

 shown conclusively that the period of heat-vibrations is not 

 affected by the state of aggregation of the molecules of the heated 

 body ; that is to say, whether the substance be in the gaseous, 

 the liquid, or perhaps the solid condition, the tendency of its 

 molecules to vibrate according to a given period remains un- 

 changed. The force of cohesion binding the molecules together 

 exercises no effect on the rapidity of vibration. 



I had arrived at the same conclusion from theoretical consi- 

 derations several years ago, and had also deduced some further 

 conclusions regarding the nature of heat-vibrations, which seem 

 to be in a measure confirmed by the experimental results of Prof. 

 Tyndall. One of these conclusions was, that the heat-vibration 

 does not consist in a motion of an aggregate mass of molecules, 

 but in a motion of the individual molecules themselves. Each 

 molecule, or rather we should say each atom, acts as if there were 

 no other in existence but itself. Whether the atom stands by 

 itself as in the gaseous state, or is bound to other atoms as in 

 the liquid or the solid state, it behaves in exactly the same 

 manner. The deeper question then suggested itself, viz. what 

 is the nature of that mysterious motion called heat assumed by 

 the atom ? Does it consist in excursions across centres of equi- 

 librium external to the atom itself? It is the generally received 

 opinion among physicists that it does. But we think that the 

 experimental results arrived at by Prof. Tyndall, as well as some 

 others which will presently be noticed, are entirely hostile to such 



* Communicated by the Author. 



