IJCt PRODUCTION OF SOUND IN VAPOUR, 



after it is depressed by the elasticity of tlie vapour formed. 

 If we then raise or lower the external level of the mercury, 

 the interior column will rise or iall exactly as iTiuch iu the 

 tube; and thus, according as the space reniainin"- at 'he 

 top of the tube is diminished or incieased, a part of the va- 

 pour will be ])recipituted, or fresh vapour will be formed : 

 but, the temperature remaining the same, the elasticity will 

 not alter. 

 Vibrations of a Now let us suppose, that a sonorous body begins to vi- 

 «^3ioTOUfc y YiYutt; in such a. medium ; each of its vibrations will diminish 

 the space in one direction, and increase it in the opposite. 

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

 duced to the liquid state, and on the other a small quantity 

 of liquid will assume the state of vapour. These condensa- 

 tions and dilatations will take placeclose tothe sonorous body 

 in the small extent of its vil>ration3, but will not be pro- 

 duced beyond this. Thus the motion will not be propa- 

 gated through the rest of the fluid mass, and consequently 

 the sound will not be transmitted. 

 Ifwesuppose Let us next suppose, that the sonorous body, in com- 

 these to disen- pressing the vapour by its rapid vibrations, disengages from 

 it mechanically a certain quantity of heat. TSus supposi- 

 tion is by no means improbable, for we know, that vapour 

 gives out a great deal of heat in its condensation. The va- 

 pour of water, fop example, according to the experiments of 

 Watt, in returning to the liquid state gives out a quantity 

 of heat, that is capable of raising the temperature of the 

 fvunj should water thus produced to 5^25° [977° ^O- I^ ^^ ^'^^^ ^^^^ ^'"■' 

 bepTodocev-n cumstance into consideration, the effects of the sonorous 

 body on vapour will no longer be the same : the portions it 

 compresses will preserve their elastic state, notwithstanding 

 the diminution of the space, in consequence of the heat 

 c^volved, which instantly increases their elasticity. On the 

 contrary, in the portion dilated the diminution of tempera- 

 ture, preventing a new evaporation, diminishes the elasti- 

 city. The phenomena produced near the sonorous body 

 >, therefore are of the same nature, as if the vapour became a 

 permanent gas. There will be successive and momentary 

 augmentations and dinrjinutions of eln^ticity, the effect of 

 which vvill be transriiittcd step by step throughout the whole 



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