1885.] third order in an isotropic elastic medium. 309 



This result is not quite in accordance with Art. 6, and might 

 suggest that the method of approximation there adopted is not 

 entirely satisfactory. 



It will be observed that the present article answers the doubt 

 raised as to the nature of the t which appeared as a factor of the 

 amplitude in Arts. 5 and 6 ; it really marks a breaking up of the 

 wave motion. 



13. The above remarks do not pretend to be a complete dis- 

 cussion of the higher terms in the equations for plane wave 

 motion in an elastic medium ; they are merely intended to point 

 out that interesting results closely bearing on the undulatory 

 theory of light may possibly be deducible from these terms. That 

 these terms would lead to some formula resembling Cauchy's has 

 been suggested, but it does not appear that any investigation of 

 these higher terms has hitherto been published. If these higher 

 terms really explain the dispersion of light in a transparent 

 medium/then the refractive index ought to vary with the intensity. 

 This result seems so contrary to all previous experience that it is 

 perhaps sufficient to justify our rejecting such an explanation 

 of dispersion. 



(3) Preliminary note on the theory of explosions. By E. Threl- 

 FALL, B.A. 



An attempt was made to account for some of the anomalous 

 effects observed in explosions by Sir Frederick Abel. 



In the well-known experiments with detonators composed of 

 various explosives, Abel was led to imagine that the apparent 

 selective efficiency of chosen substances when applied to explode 

 one another might be accounted for on a hypothesis of " synchro- 

 nous vibrations." It was pointed out that this hypothesis can 

 have no possible physical meaning, unless the vibrations be sup- 

 posed to take place in the ether ; and an explanation was sought 

 in the behaviour of the products of explosion as regards their 

 motion in air, and in water. For this purpose the explosives 

 were treated in groups arranged with respect to their supposed 

 time of decomposition, and it was shewn that much would depend 

 on the method of "break up" of the volume of gas set free by 

 the explosion. The various ways in which the energy of an 

 explosion might be transmitted through fluids were enumerated, 

 and some stress was laid on the effects to be expected if the 

 conditions of explosion were such as to lead to the production of 

 vortex rings. 



The necessity of clearly defining the meaning of the phrase 

 " violence of explosion " was pointed out, and it was shewn that 



