57 S On the Production of Sounds in Vapours. 



purpose it is sufficient to introduce into a barometer a small v 

 quantity of any liquid, and to measure the height at which 

 the mercury stops after being depressed by the elastic force 

 of the vapour that is formed. If then the external surface 

 of the mercury is either elevated or depressed, the interior 

 column will rise or fall in the lube precisely in the same 

 proportion ; and according as the space which remains in 

 the top of the tube is diminished or increased, a part of the 

 vapour will precipitate itself or rise afresh ; but the tempe- 

 rature remaining the same, no variation will appear in the 

 clastic force. 



Now, supposing a sonorous body vibrates in a simi- 

 lar medium, each of its oscillations will diminish, in 

 one sense, the space, and will augment it in the other* 

 Thus on one side there will be a small quantity of vapour 

 which will pass into the liquid state; and on the contrary, 

 a small quanitiy of liquid that will assume the slate of va- 

 pour. These. condensations and expansions will take place 

 very near the sonorous body, in the immediale vicinity of 

 the vibrations ; but they will not be further extended. Thug 

 the impulse will not be exerted on the remaining fluid mass, 

 and consequently tlie sound will not be transmitted. Now 

 let us suppose that the sonorous bodv, compressing the 

 vapour by its rapid vibralions, mechanically disengages 

 from it a certain quantity of heat. This supposition is not at 

 all improbable, for it is well known that much heat is emit- 

 ted during the condensation of vapour. For instance, the 

 vapour of water, according to the experiments of Watt, 

 sets at liberty, v.hilst passing from the aeriform to the fluid 

 state, heat sufficient to raise the mass of water thus formed 

 to 525 of the thermometer centesimal. Paying attention to 

 this circumstance, the effects of a sonorous body on vapour 

 are not the same j the compressed' portions maintain the 

 stale of an eicistic fluid in spite of the diminution of space, 

 on account of the liberated heat, which gives them a mo- 

 inentary increase of pov.er. On the contrary, in the ex- 

 panded portion, the decrease of temperature prevents a new 

 formalion of vapour, and occasions a diminution of elasti- 

 city. The phasnomcna which are produced near the sono- 

 rous body, are, then, of the same nature as if the vapour be- 

 came a permanent gas. They consist of increments and 

 diminution of elasticity successively and momentarily pro- 

 duced, spreading their efiects from strata to strata through 

 the whole fluid mass, so as to permit ihe production and 

 propagation of sound. 



Experinjents, therefore, on the production of sound in 



vanour 



I 



