ON THE MEASUllEi: AND THE IfATURE OF HEAT. 65S 



parts of bodies, by percussion, by friction, or by the destruction of the 

 equilibrium of cohesion and repulsion, and by a change of the conditions ori 

 which it may be restored, in consequeiice of combustion, or of any other 

 chemical change. It is remarkable that the particles of fluids, which are 

 incapable of any material change of temperature from mutual friction, have 

 also very little power of communicatii>g heat to each other by their imme- 

 diate action, so that there may be some analogy, in this respect, between 

 the communication of heat and its mechanical excitation. 



The effects of heat on the cohesive and repulsive powers of bodies, have 

 sometimes been referred to the centrifugal forces and mutual collisions of the 

 revolving and vibrating particles ; and the increase of the elasticity of aeri- 

 form 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 imme- 

 diate 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 resem- 

 ble tloose of sound, since every sounding body is in a state of vibration^ and 



