PHYSICS. 



By George F. Barkee, M. D., 



Professor of Physios in the University of Pennsylvania, Philadelphia, 



GENERAI.. 



The lectures on Molecular Dynamics, given by Prof. Sir William 

 Thomson at the Johns Hopkins University, Baltimore, in October, 1884, 

 mark an 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 an 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 diffi- 

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

 of explaining the dependence of velocity of propagation on period of 

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

 phenomena of reflection and refraction, and the fourth those of double 

 refraction. As to the first difficulty, 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- 

 ThomsoQ) spring and shell molecule to represent it. As to the sether, 

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

 perfect mobility, since the question is merely one of time. The aether 

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

 millionth 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 thiug that the 

 luminous vibrator consists in seems to me to be a sudden iuitiatiou of 

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

 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, must 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 <iaartz peb- 



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