112 PROCEEDINGS OF SECTION A. 



as the wave-length increases. Direct comparison with the spectrum 

 from a spark discharge shows that below X = 2350 and A = 2630 

 all lines have their counterparts in the polar lines in the arc, but 

 with increasing wave length the arc becomes richer in median lines, 

 some of which now correspond to lines from the spark discharge, 

 and the polar lines decrease, as ah'eady stated, in number and 

 intensity. 



The origin of the polar lines and the bearing of pressure, density, 

 temperature, and potential gradient upon the phenomenon are dis- 

 cussed. The distinctive character of the polar lines should assist in 

 the resolution into series of the iron arc spectrum. In the copper 

 arc spectrum there are differences between the lines, which admit of 

 classification into polar arc lines (those strongest at the poles of an 

 arc) and polar spark lines (those strongest at the poles of a spark 

 discharge). These behave differently under different external condi- 

 tions. * 



In the iron arc in the extreme ultra-violet the median lines are 

 diffuse and nebulous, the polar lines sharp. Instances are given of 

 median lines losing in intensity in the arc when polar lines appear 

 near them. 



11.— ELASTIC SOLID ETHER, WITH TWO MODULI, SATISFYING 

 MacCULLAGH'S crystalline OPTICAL CONDITIONS. 



By PROFESSOR A. MCAULAY, M.A., University of Tasmania. 



12.— ON THE RADIUM CONTENT OF CERTAIN IGNEOUS ROCKS 

 FROM THE SUB-ANTARCTIC ISLANDS OF NEW ZEALAND. 



By C. COLERIDGE FARR, D.Sc, and D.' C. H. FLORANCE, M.A. 



13.— RECENT EXPERIMENTS ON THE VISCOSITY^ OF WATER. 

 By RICHARD HOSKING, B.A. (Camb.), B.Sc. (Sydney). 



14.— THE SPECTRUM OF SILVER GIVEN BY' A CARBON-TUBE 



FURNACE. 



By W. G. DUFFIELD, D.Sc, F.R.A.S. 



15.— THE LAWS OF :M0BILITY AND DIFFUSION OF THE IONS 

 FORMED IN GASEOUS MEDIA. 



By E. M. WELLISCH, M.A., Emmanuel College, Cambridge, Eiuj_ 



Abstract. 

 Expressions have been deduced from the kinetic theory of gases 

 for the mobility and coefficient of dift'usion of an ion, allowance being 

 made for the increase in collision frequency due to the polarisation of 

 the neutral molecules hj the charge associated with the ion. This 

 charge is shown to be replaceable, as far as collisions are concerned, 



* Duffield, "Effect of Pressure on Arc Spectra, No. 2 Copper." — Phil. Trans. 

 Roy. Society. 



