Vibration of Heated Metals. 145 



dies, yet the very soft mucous texture which covers them appears 

 to be intended for the very purpose of preventing or extinguish- 

 ing any tremor which might arise. Could we suppose a flute, 

 or any wind-instrument, constructed of inelastic earthenware, 

 we should have a perfect source of that aerial vibration to which 

 I am now alluding, and such I conceive the organs of the animal 

 voice to be. 



Thirdly, When air, anyhow thrown into tremors, impinges 

 upon an elastic solid, and throws it into vibration, the tremors of 

 the solid body are in their turn communicated to the air in con- 

 tact, and the sonorous pulses may be considered as a complex 

 modification, resulting from the intermixture or semi-combination 

 of the two sets of tremors. 



The acoustic tremors of ordinary wind-instruments are of 

 this description. They are finely illustrated by the wild ./Eolian 

 tremulous notes emitted by the cylindrical glass-tube in which 

 a very slender jet of hydrogen gas is made to burn ; also by the 

 very loud notes issuing from the gas-lamp furnace now employed 

 in chemical experiments, when a chimney eight or ten inches 

 high is placed upon them. 



These three are the more common methods of exciting 

 sound-causing pulses ; but, in our experiment, the source of the 

 sonorous undulations of the air appears to be different. In it we 

 have a solid thrown into vibration by the peculiar mode of the 

 passage of caloric from one mass of metal into another, and ne- 

 cessarily occasioning tremors in the contiguous air ; and we have, 

 at the same time, the air in undulatory movement, induced by 

 flowing in a current through the channels afforded by the rough- 

 ness of the block or bar ; but to produce a soniferous undulation, 

 neither of these singly is sufficient. It is the combination which 

 causes the result. 



This appears to me to be demonstrated by the facts, 1st, 



