1917] Recent Developments of Molecular Physics 



WEEKLY EVENING MEETING, 



Friday, March oO, 1917. 



CoLOXEL Ed^iond H. Hills, C.M.G. R.E. D.Sc. F.R.S., 

 Secretary and Vice-President, in the Chair. 



J. H. Jeans, M.A. E.R.S. 



Recent Developments of Molecular Physics. 



As the subject of my discourse this evening I propose to take the 

 surprising developments in the fundamental conceptions of physics 

 \Yhich have been forced upon us in the last few years. In the closing 

 year of last century, Lord Kelvin delivered a Friday evening discourse, 

 taking as his title " Nineteenth Century Clouds over the Dynamical 

 Theory of Light and Heat." He said : " The beauty and clearness of 

 the dynamical theory which asserts light and heat to be two modes of 

 motion is at present obscured by two clouds." The cloud over the 

 dynamical theory of light centred round the question of the motion 

 of the earth through the ether ; that over the dynamical theory of 

 heat was concentrated about the famous theorem of Equipartition of 

 Energy. In the seventeen years which have elapsed since then, the 

 attempt to remove these clouds has led to the introduction of two 

 new principles — or perhaps it is better to speak of them as theories, 

 since they are both still in a tentative stage. The Theory of Rela- 

 tivity has been introduced to remove the first cloud, and the Theory 

 of Quanta to remove the second. 



These are, perhaps, the two most revolutionary theories that have 

 been seriously considered by science for some centuries. They are 

 revolutionary especially because they involve a new philosophical and 

 metaphysical outlook which implies a complete break from that 

 formerly held. It need hardly be said that the new theories are still 

 very far from having achieved universal acceptance, but whether 

 they ultimately prove to be true or not, the mere consideration of 

 them has beyond dispute done a great deal towards suggesting a way 

 out of the impasse in which molecular physics found itself at the end 

 of last century. The two clouds of which Lord Kelvin spoke are not 

 yet completely dissolved, but rays of light are appearing here and 

 there, and the vista beyond, interpret it in any way we will, is 

 certainly one of the most interesting and fascinating that has ever 

 spread itself before the physicist. 



