THE MEASURES AND THE NATURE OF HEAT. 503 



sometimes been referred to the centrifugal forces and mutual collisions of 

 the revolving and vibrating particles : and the increase of the elasticity of 

 aeriform fluids has been very minutely compared with the force which 

 would be derived from an acceleration of these internal motions. In solids 

 and in liquids, however, this increase of elasticity is not observable, and 

 the immediate effect of heat diminishes not only the force of cohesion, but 

 also in some degree, that of repulsion, so that these vibrations, if they 

 exist, must derive their effect on the corpuscular forces from the alterations 

 which they produce on the situation of the particles, with respect to the 

 causes of these forces. 



The different chemical effects of heat and light are far from furnishing 

 any objection to this system ; it is extremely easy to imagine the attraction 

 between two or three bodies to be modified by the agitations, into which 

 their particles are thrown. If certain undulations be capable of affecting 

 one of the three bodies only, its cohesion with both the others may be 

 weakened, and hence their mutual attraction may be comparatively 

 increased ; and from various combinations of such differences, in the 

 operation of different kinds of heat and light, a great diversity of effects of 

 a similar kind may be derived. 



If heat, when attached to any substance, be supposed to consist in 

 minute vibrations, and when propagated from one body to another, to 

 depend on the undulations of a medium highly elastic, its effects must 

 strongly resemble those of sound, since every sounding body is in a state 

 of vibration, and the air, or any other medium, which transmits sound, 

 conveys its undulation to distant parts by means of its elasticity. And we 

 shall find that the principal phenomena of heat may actually be illustrated 

 by a comparison^with those of sound. The excitation of heat and sound 

 are not only similar, but often identical ; as in the operations of friction 

 and percussion : they are both communicated sometimes by contact and 

 sometimes by radiation ; for besides the common radiation of sound 

 through the air, its effects are communicated by contact, when the end of 

 a tuning fork is placed on a table, or on the sounding board of an instru- 

 ment, which receives from the fork an impression that is afterwards propa- 

 gated as a distinct sound. And the effect of radiant heat, in raising the 

 temperature of a body upon which it falls, resembles the sympathetic 

 agitation of a string, when the sound of another string, which is in unison 

 with it, is transmitted to it through the air. The water, which is dashed 

 about by the vibrating extremities of a tuning fork dipped into it, may 

 represent the manner in which the particles at the surface of a liquid are 

 thrown out of the reach of the force of cohesion, and converted into 

 vapour ; and the extrication of heat, in consequence of condensation, may 

 be compared with the increase of sound produced by lightly touching a 

 long cord which is slowly vibrating, or revolving in such a manner as to 

 emit little or no audible sound ; while the diminution of heat, by expan- 

 sion, and the increase of the capacity of a substance for heat, may be 

 attributed to the greater space afforded to each particle, allowing it to be 

 equally agitated with a less perceptible effect on the neighbouring particles. 

 In some cases, indeed, heat and sound not only resemble each other in 



