PHYSICS. 



By George 1'\ Bakkek, M. D., 



ProfcHHor of rhij^sici in the Uuiversilii of Pcnust/lmiiia, I'lnlurlelpliia. 



GENERAL. 



The lectures on Moleculjir Dynamics, given by Frof. Sir Williaui 

 Tliomsou at the Johns Uopkins University, JJaltinjore, in October, 1884, 

 mark au epoch in the higher physical instruction ot this country. They 

 were devoted to a discussion of the wave theory of light, considering 

 (1) the propagation of a disturbance through au elastic medium, (2) the 

 character of molecular vibration, and (3) the influence of molecules on 

 the propagation of waves. The lectures dealt largely with the difli- 

 culties of the wave theory, the first enumerated being the diificulty 

 of explaining the dependence of velocity of propagation on period of 

 vibration, the second the properties of the cethcr, the third certain 

 ])henomena of reflection and refraction, and the fourth those of double 

 refraction. As to the first difiiculty, he devoted a very considerable 

 space to Helmholtz's theory, which ascribes dispersion to a compound 

 structure of material molecules according to which they have a natural 

 period of vibration, suggesting an ingenious mechanical (Helmholtz- 

 Thomson) spring and shell molecule to represent it. As to the lether, 

 he has no diflQcidty in reconciling its almost perfect rigidity with almost 

 perfect mobility, since the (luestion is merely one of time. The lether 

 may be highly elastic for vibrations executed in the 100 or 1,000 million 

 millioutli of a second, but highly mobile to bodies going through it at 

 the rate of 20 miles a second. As to the manner in which the molecule 

 imparts its motion to the ether, he says: -'The kind of tiling that the 

 luminous vibrator consists iu seems to me to be a sudden initiation of 

 a set of vibrations and a sequence of vibrations from that initiation 

 which will naturally become of smaller and smaller amplitude. Why 

 a sudden start? Because I believe that the light of the natural flame 

 or of the arc light, or of any other known source of light, miust be the 

 result of sudden shocks from a number of vibrators. Take the light 

 obtained by striking two quartz pebbles together. You have all seen 

 that. There is one of the very simplest sources of light. What sort 

 of a thing can the light be that proceeds from striking two quartz peb- 



r>TT 



U. iMis. l.j ;j7 



