Mr. T. S. Thomson on the Diffusion of Gases. S47 



" Some authors ascribed this fact to the cessation of the 

 humming of insects, the singing of birds, and the action of the 

 wind on the leaves of trees; but M. Humboldt justly main- 

 tains that this cannot be the cause of it on the Orinoco, where 

 the buzz of insects is much louder in the night than in the 

 day, and where the breeze never rises till after sunset. Hence 

 he was led to ascribe the phaenomenon to the perfect trans- 

 parency and uniform density of the air, which can exist only 

 at night after the heat of the ground has been uniformly dif- 

 fused through the atmosphere. When the rays of the sun 

 have been beating on the ground during the day, currents of 

 hot air of different temperatures, and consequently of different 

 densities, are constantly ascending from the ground and mix- 

 ing with the cold air above. The air thus ceases to be a ho- 

 mogeneous medium; and every person must have observed 

 the effects of it upon objects seen through it, which are very 

 indistinctly visible, and have a tremulous motion as if they 

 were dancing in the air. The same effect is perceived when 

 we look at objects through spirits and water that are not per- 

 fectly mixed, or when we view distant objects over a red-hot 

 poker or over a flame. In all these cases the light suffers re- 

 fraction in passing from a medium of one density into a me- 

 dium of a different density ; and the refracted rays are con- 

 stantly changing their direction as the different currents rise 

 in succession. Analogous effects are produced when sound 

 passes through a mixed medium, whether it consists of two 

 different media, or of one medium where portions of it have 

 different densities. As sound moves with different velocities 

 through media of different densities, the wave which produces 

 the sound will be partly reflected in passing from one medium 

 to the other, and the direction of the transmission wave 

 changed ; and hence in passing through such media, different 

 portions of the wave will reach the ear at different times, and 

 thus destroy the sharpness and distinctness of the sound. 

 This may be proved by many striking facts. If we put a bell 

 in a receiver containing a mixture of hydrogen gas and atmo- 

 spheric air, the sound of the bell can scarcely be heard." 



With all due deference to so great an authority as Sir David 

 Brewster, I cannot admit the analogy between the non-exist- 

 ence of sound in a mixture of hydrogen gas and atmospheric 

 air, and the interruptions to hearing and vision produced by 

 the refraction of sound and light "in passing from a medium 

 of one density into a medium of a different density." Surely 

 when hydrogen gas and atmospheric air are mixed in a re- 

 ceiver, there cannot exist media of different densities? In 

 accordance with the law of the diffusion of gases, the arrange- 



