BY COLOURED MEDIA. 493 



laws of structure competent to apply to this or that phae- 

 nomenon. I will, however, mention one or two facts in 

 acoustics which appear to me strongly illustrative of cor- 

 responding phenomena in the propagation of light. 

 The first of these is the impeded propagation of sound in 

 a mixture of gases differing much in elasticity as com- 

 pared with their density. The late Sir J. Leslie's experi- 

 ments on the transmission of sound through mixtures of 

 hydrogen with atmospheric air sufficiently establish this 

 remarkable effect. It would be desirable to prosecute 

 those experiments in larger detail, but hitherto I am not 

 aware of anybody having ever repeated them. It would 

 be interesting, for instance, to inquire whether the im- 

 pediment offered by such a mixture of gases be the 

 same for all pitches of a musical note, or not ; and how 

 far this phenomenon might be imitated by mixing actual 

 dust of a uniform size of particle, such as the dust of 

 Lycoperdon, &c. ; or aqueous fog, and how far such mix- 

 ture would affect unequally sounds of different pitches. 



(19.) The other fact in the science of acoustics which 

 I would notice as illustrative of a corresponding phseno- 

 menon in photology, is one observed by Mr Wheatstone, 

 which I have his permission to mention. In attempting 

 to propagate vibrations along wires, rods, &c., to great 

 distances, he was led to remark a very great difference 

 in respect of facility of propagation between vibrations 

 longitudinal and transverse to the general direction of 

 propagation. The former were readily conveyed with 

 almost undiminished intensity to any distance j the latter 

 were carried off so rapidly by the air, as to be incap- 



