1878.] reversal of the lines of Metallio Vapours. 161 



the air could be exhausted and other gases introduced at will. In 

 this form of tube the metals could be kept in the state of vapour 

 in the bulb for some time without obscuring the ends. The same 

 absorption lines were seen in the glass tubes as had been observed 

 in the iron tubes. The authors remarked that the density of the 

 vapour of sodium at the temperature of the iron tubes appeared 

 abnormally great, judging from the quantity of the metal which 

 was needed to fill the hot part of the tube with the vapour. As 

 soon as the vapour rose to the level of the end of the small tube 

 conveying the hydrogen, a black cloud of sodium vapour con- 

 densed by the cool hydrogen completely obscured all the light 

 from the lower part of the tube, and a smoke of soda issued with 

 the hydrogen from the open end of the tube. 



They did not succeed in getting any absorption spectra pro- 

 duced by zinc, cadmium, lead or silver, though all these, except 

 perhaps silver which gave no distinctly recognisable vapour, were 

 volatilized in the tubes. 



May 6, 1878. 

 Professoe Cayley, Vice-President, in the chair. 



A communication was read by Prof. Clerk-Maxwell, ''On 

 Boltzmanns Theorein on the average distribution of energy in a 

 system of 7naterial j^oints." 



The publication of this paper is deferred for the present. 



May 20, 1878. 

 Professor Liveing, President, in the chair. 



Pev. E. Hill, "An elementary discussion of some jwints connected 

 with the Influence of Geological changes on tJte Earth's Axis of 

 rotation." 



A paper has lately been read before the Poyal Society by 

 Mr G. H. Darwin, in which he investigates the effect which will be 



