270 



Lord Kelvin. On the 



[Feb. 13, 



free path of their molecules. When pressure is lowered these gases 

 appear to conduct less well than the diatomic gases ; and it may be 

 that their molecular constitution is less affected, if affected at all, by 

 diminution of pressure. 



The behaviour of mixtures of argon and helium at low pressures 

 is somewhat inscrutable. Why should argon convey current almost 

 to the exclusion of helium, if, indeed, visibility of its spectrum be 

 taken as an indication of conductivity F In this respect argon rather 

 resembles the diatomic gases. 



It may be objected that, in making such observations, one con- 

 sideration has been neglected. It is the nature of the electrodes. 

 This may indeed play a not unimportant part in the phenomena. It 

 is unquestionable that hydrogen and helium are both absorbed by 

 platinum, and it would appear that nitrogen, too, can be made to 

 combine, for it is removed from a vacuum tube containing it, if the 

 platinum is " splashed " on to the walls. But the experiments on 

 mixtures of gases were performed in vacuum tubes without, elec- 

 trodes, and the influence of the electrodes comes in only with experi- 

 ments on gases at atmospheric pressure, and in those in which 

 pressure was reduced. Their pressure may, however, conduce to 

 some kind of polarising influence, and the conclusions, such as they 

 are, may be vitiated by neglecting this factor. 



We trust that these somewhat rash speculations will be forgiven ; 

 the subject is a difficult one, and we have carried out the experi- 

 ments more with the view of obtaining practical help in recognising 

 the purity of argon and helium than of carrying out a research on 

 the relative conductivities of gases. There is evidently a wide field 

 for research, and we shall be glad if this description of our methods 

 of manipulation and of the somewhat meagre results which we have 

 obtained may induce others, better qualified than we, to make similar 

 investigations. 



II. "On the Generation of Longitudinal Waves in Ether." 

 By Lord Kelvin, F.R.S. Received February 10, 18%. 



In a short note published in last week's ' Nature,' of which a copy 

 is appended, I suggested an arrangement of four insulated and 

 electrified spherical conductors with their centres in one line, giving 

 rise to ethereal waves in the surrounding atmosphere, of which the 

 disturbance in the line of centres is essentially longitudinal. But at 

 any finite distance from this line there must also be laminar or dis- 

 tortional waves of the kind expressed in Maxwell's equations. The 

 object of my present communication is to show an arrangement by 

 which a large space of air is traversed by pressural disturbance, or 

 by Avaves essentially longitudinal, or by condensational- raref actional 



