164: ANNUAL OF SCIENTIFIC DISCOVERY. 



of the previously condensed moisture should evaporate and cool the 

 surface of the body. Results perfectly similar to those obtained with 

 vapor of water were obtained by using vapor of alcohol, or of ether, 

 or other vapors. Generalizing : the most various vapors condense on 

 the surface of solid bodies in such a quantity as to cause appreciable 

 elevation of temperature. From this it follows that, at all times, 

 there is at the surface of solid bodies a layer of condensed vapor, 

 which is larger or smaller according to the hygrometric state of the 

 atmosphere. Under some conditions, this will, without doubt, exer- 

 cise a by no means unimportant influence. 



OX THE NATURE OF HEAT-VIBRATIONS. 



Prof. Tyndall, in his researches on radiant heat, has shown that 

 the period of heat-vibrations is not affected by the state of aggrega- 

 tion of the molecules of the heated body ; that is to say, whether 

 the substance be in the gaseous, the liquid, or perhaps the solid con- 

 dition, the tendency of its molecules to vibrate according to a given 

 period, remains unchanged, the force of cohesion binding the mole- 

 cules together, exercises no effect on the rapidity of vibration. Mr. 

 James Croll, in a communication to the Philosophical Magazine, states 

 that he has 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 them- 

 selves. 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 assumed by the atom called heat ? Does it consist 

 in excursions across centers of equilibrium 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. Tyn- 

 dall, as well as some others which will presently be noticed, are entire- 

 ly hostile to such an opinion. The relation of an atom to its center 

 of equilibrium depends entirely on the state of aggregation. Xow 

 if heat-vibrations consist in excursions to and fro across these centers, 

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

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



*^j d 



other words, the more (irmly its molecules are bound together, the 

 higher is the pitch. Two harp-chords struck with equal force will 

 vibrate with equal force, however much they may differ in the rapidity 

 of their vibrations. The r/.s- viva of vibration depends upon the 

 force of the stroke ; but the rapidity depends, not on the stroke, but 

 on the tension of the cord. 



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

 or atoms across centers of equilibrium follows also, as a necessary 

 consequence, from the fact that the real specific heat of a body re- 

 mains unchanged under all conditions. All changes in the specific 



