IN'FLUEXCE OF LIGHT OX THE PROPAGATION OF SOUND. ^V 



phenomenon exhibited by the paper, the motion of which hut this will 

 depended on the tremuUition of the air of the room. Is the erfect^on^the ^ 

 atmospheric air more dense on the appearance of light than api:>ai^iu6. 

 in darkness ? Is this greater density of the air, or of the 

 elastic fluid that is subservient to the propagation of sound, 

 the effect of aeriform substances kept in this state through 

 the medium of liglit? On this hypothesis vve must suppose, 

 that a greater density may take place without increasing the 

 weight of the air; and vve might ascribe the increase in the 

 propagation of sound to a greater elasticity in the fluids dif- 

 fused through the atmosphere. This would confirm the Pile-tley'« 

 opinion of Dr. Priestley, who said, that souad was propa- '^P'"*^"' 

 gated in different gasses ifj the ratio of their density. Ent 

 the rays of the solar light ave inseparable from the calorific 

 rays: their presence therefore, by raising the temperature, 

 muft produce a dilatation in the circumambient air, that 

 seems to exclude the hypothesis of condensation. Besides, 

 it is proved by the experiments of Mr. Perolle, inserted ia 

 the 3d vol. of the Memoirs of the Royal Academy of Sci- 

 ences of Turin, that the propagation of sound is affected, 

 not merely by the density of the different gasses it traverses^ 

 but by their nature. 



]Mr. Perolle having placed an alarum watch in a jar Perolle fnind 

 which he filled successively with different gasses, retired '''^'.'^"■'"'^ was 

 gradually from the apparatus, stopping at the point where of ti.e densiiy 

 the sound was no longer audible to him. Proceeding in °^ ^^^ i"**- 

 this.manner he found, that the weight of a cubic foot of 

 <;ach of the gasses tried being, 



Carbonic acid gas. •• •• 1030, 



Oxigen gas 765, 



Atmospheric air 7-^0, 



Nitrous gas 6<j8, 



Hidrogeu gas 7^2, 



the propagation of sound followed an order not always ana- 

 logous to that of the density. It was as follows. 



1b 



