656 LECTURE txr. 



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 prin- 

 cipal 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 com- 

 municated by contact, when the end of a tuning fork is placed on a table, or 

 on the sounding board of an instrument, which receives from the fork an 

 impression that is afterwards propagated 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 convert- 

 ed 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 chord which is slowly vibrating, or revolving in such a manner as to 

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

 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 their operations, but 

 produce precisely the same effects; thus, an artificial magnet, the force of 

 which is quickly destroyed by heat, is affected more slowly in a similar 

 manner, when made to ring for a considerable time; and an electrical jar 

 may be discharged, either by heating it, or by causing it to sound by the 

 friction of the finger. 



All these analogies are certainly favourable to the opinion of the vibratory 

 nature of heat, which has been sufficiently sanctioned by the authority of 

 the greatest philosophers of past times, and of the most sober reasoners of 

 the present. Those, however, who look up with unqualified reverence ta 

 the dogmas of the modern schools of chemistry, will probably long retain % 



