Mr. James Croll on the Nature of Heat Vibrations. 347 



an opinion. The relation of an atom to its centre of equilibrium 

 depends entirely on the state of aggregation. Now if heat- 

 vibrations consist in excursions to and fro across these centres, 

 then the period ought to be affected by the state of aggregation. 

 The higher the tension of the atom in regard to the centre, the 

 more rapid ought its movement to be. This is the case in regard 

 to the vibrations constituting sound. The harder a body becomes, 

 or, in other words, the more firmly its molecules are bound 

 together, the higher is the pitch. Two harp-cords struck with 

 equal force will vibrate with equal force, however much they may 

 differ in the rapidity of their vibrations. The vis viva of vibra- 

 tion depends upon the force of the stroke; but the rapidity 

 depends, not on the stroke, but upon the tension of the cord. 



That heat- vibrations do not consist in excursions of the mole- 

 cules or atoms across centres of equilibrium, follows also as a 

 necessary consequence from the fact that the real specific heat of 

 a body remains unchanged under all conditions. All changes 

 in the specific heat of a body are due to differences in the amount 

 of heat consumed in molecular work against cohesion or other 

 forces binding the molecules together. Or, in other words, to 

 produce in a body no other effect than a given rise of tempera- 

 ture, requires the same amount of force, whatever may be the 

 physical condition of the body. Whether the body be in the 

 solid, the fluid, or the gaseous condition, the same rise of tem- 

 perature always indicates the same quantity of force consumed 

 in the simple production of the rise. Now if heat-vibrations 

 consist in excursions of the atom to and fro across a centre of 

 equilibrium external to itself, as is generally supposed, then the 

 real specific heat of a solid body, for example, ought to decrease 

 with the hardness of the body, because an increase in the strength 

 of the force binding the molecules together would in such a case 

 tend to favour the rise in the rapidity of the vibrations. 



These conclusions not only afford us an insight into the hidden 

 nature of heat-vibrations, but they also appear to cast some light 

 on the physical constitution of the atom itself. ■ They seem to 

 lead to the conclusion that the ultimate atom itself is essentially 

 elastic. For if heat-vibrations do not consist in excursions of 

 the atom, then it must consist in alternate expansions and con- 

 tractions of the atom itself. This again is opposed to the ordi- 

 nary idea that the atom is essentially solid and impenetrable. 

 But it favours the modern idea, that matter consists of a force of 

 resistance acting from a centre. 



