On the Production of Sounds in Vapours. 279 



vapour are verv proper to decide the question, whether heat 

 is really disengaged by the effect of the vibrations of sono- 

 rous bodies in ai) aeriform mediuit), as weseegcnerany takes 

 place on all rapid coniprcssions. To this test, and .it is a 

 decisive; test, ihay the inj^enious idea of M. La Place be sub- 

 mitted, bv which he has found means to reconcile the ma- 

 thematical theory of the propagation of sound in air with 

 expL-rimental results, in which the disengaged heat was 

 dulv attended to ; for, if the effect which he infers doesnbt 

 realiv take placej the vibrations of sonorous bodies in va- 

 pours ought absolutely to produce no sound ; and if any 

 is produced, it can be considered only as the sole effect of 

 the liberated heat. 



luflueuced by this motive, I made on the subject some 

 experiments which were attended with evident success ; and 

 I have since repeated them in a more complete manner at 

 Arcueil with my friend A'medee BerthoHet. 'M. Berthotlet 

 and M. La Place were present at those experiments, and 

 fully satisfied themselves of the truth of the iaets which I 

 proceed to relate. 



We used a glass balloon of the capacity of 36 litres ; its 

 orifice was closed by a perfectly tight stop-cock, so that ah 

 exhaustion might be made, and preserved good. To this 

 another mifflit be connected, that, by pouring a liquid into 

 the intermediate hollow and closing both cocks, this por- 

 tion of liquid might be introduced into the interior of the 

 balloon wuliout any danger of admitting the least external 

 air. Lastly, the sonorous body was a small hell suspended 

 within bv a very fine cord tied to the lower stop-cock. 



First, a vacuum v^'as formed in the interior of the appa- 

 ratus with the greatest care, and even the largest part of 

 the hygromctric water of the balloon, which was always 

 very dry, was abstracted. Then holding the balloon by the 

 stop-cock, the bell vvas set in motion, satisfying ourselves 

 that the blows foil with force on the metal ; and whatever 

 attention was paid, or however near we wer6 to the balloon 

 itself, it was absolutely impossible to distinguish any per- 

 ceirtible sound : therefore sound is not sensible in the va- 

 cuum ; a fact consi ♦. tly in harmony with the experiments 

 of l4awksbee, and of all other natural philosophers. 



Then proceeding as 1 have descrttied, a small quantity of 

 water was introduced, part of which rose in vapour. Im- 

 mediately the sound became perceptible; yet the density of 

 this vapour was very little, the temperature being no more 

 ihanlyof the therujometer centesimal : to increase it an 

 ei:ccs8 of water was added, and the balloon was carried into 

 S 4 a green- 



