Method of Measuring Electrolytic Conductivity. 19 



dissociation of the molecules. According to the law of the 

 inverse fourth power, apparent departures from Boyle's law 

 on account of pure molecule r attraction of solid walls for the 

 gas cannot be of amount at present detectable in ordinary 

 vessels, but are possible in small interspaces between solids, as 

 in powders or porous bodies, if the interspaces are small 

 enough. As the kinetic theory gives no hint that the kinetics 

 of molecules should alter with rarefaction of gases, except 

 towards stricter and stricter rigorousness of Boyle's law, and 

 as the theory of molecular force requires a very small effect 

 for the walls of ordinary vessels, we may say that theoretically 

 there is no cause to expect real or apparent departures from 

 Boyle's law at low pressures on general grounds, and that when 

 such are obtained experimentally they are due to special causes, 

 such as a chemical or quasi-chemical attraction of glass for 

 water or moist C0 2 , or dissociation or combination, as in the 

 case of oxygen (to be proved in a separate paper), and therefore 

 in that of air. The case of hydrogen may be regarded as the 

 typical one hitherto investigated that is free from special con- 

 ditions, and the whole tenonr of the experimental evidence 

 relating to it is to justify the laws of Boyle and Charles for it 

 when rarefied. 



The direction which further experimental inquiry should 

 take is obviously that of varying the conditions more widely 

 than hitherto. 



Although the amount of direct experimental evidence in 

 favour of Boyle's law in rare gases has been shown to be small, 

 it seems to be cogent ; and in my next paper, on " Two New 

 Pressure-Gauges,' ' I shall show that it is well supported by 

 indirect evidence obtainable from Crookes's experiments on 

 High Vacua. 



Melbourne, Aug. 1896. 



III. A Satisfactory Method of Measuring Electrolytic Conduc~ 

 tivity by means of Continuous Currents. By Prof. W. 

 Stroud, D.Sc, M.A., and J. B. Henderson, B.Sc* 



THE devices for eliminating or reducing the disturbing 

 effects of polarization in the measurement of the con- 

 ductivity of electrolytes are very numerous. Wheatstone, 

 Horsford, Wiedemann, Beetz, Paalzow, Ewing and Mac- 

 gregor, Bouty, and others have all experimented on this 

 subject. It will, however, be universally conceded that 



* Communicated bv the Physical Societv : read Oct. 30 ; 1896. 

 C2 



