1881.] On the Viscosity of Gases at High Exhaustions. 457 



bounded, as regards its physical state, on the one side by the sub- 

 gaseous or liquid condition, and on the other side by the ultra - 

 gaseous condition. A gas assumes the former state when condensed 

 by pressure or cold, and it changes to the latter state when highly 

 rarefied. Before actually assuming either of these states there is a 

 kind of foreshadowing of change, with partial loss of gaseity. 

 When the molecules, by pressure or cold, are made to approach each 

 other more closely, they begin to enter the sphere of each other's at- 

 traction, and therefore the amount of pressure or cold necessary to 

 produce a certain density is less than the theoretical amount by the 

 internal attraction exerted on each other by the molecules. The 

 nearer the gas approaches the point of liquefaction the greater is the 

 attraction of one molecule to another, and the amount of pressure re- 

 quired to produce any given density will be proportionally less than 

 that theoretically required by a " perfect " gas. 



The Ultra-gaseous State of Matter. 



After some theoretical considerations respecting the viscosity of 

 gases, the author concludes with the detailed statement of his theory 

 of the existence of an ultra- gaseous state of matter. 



A consideration of the curves of the gases, especially hydrogen, 

 which are given in the paper, will confirm the supposition that a gas, 

 as the exhaustions become extreme, gradually loses its gaseous charac- 

 teristics, and passes to an ultra-gaseous state. 



An objection has been raised touching the existence of ultra-gaseous 

 matter in highly exhausted electrical tubes, that the special phenomena 

 of radiation and phosphorescence which the author has considered 

 characteristic of this form of matter can be made to occur at much 

 lower pressures than that which exhibits the maximum effects. For 

 the sake of argument let us assume that the state of ultra gas with 

 its associated phenomena is at the maximum at a millionth of an at- 

 mosphere. Here the mean free path is about 4 inches long, sufficient 

 to strike across the exhausted tube. But it has been shown by many 

 experimentalists that at exhaustions so low that the contents of the 

 tube are certainly not in the ultra- gaseous state, the phenomena of 

 phosphorescence can be observed. This circumstance had not escaped 

 the author's notice. In his first paper on the " Illumination of Lines 

 of Molecular Pressure and the Trajectory of Molecules," * the author 

 drew attention to the fact that a molecular ray producing green phos- 

 phorescence can be projected 102 millims. from the negative pole 

 when the pressure is as high as 0*324 millim., or 427 M. In this case 

 the mean free path of the molecules is 0*23 millim. ; and it is not sur- 

 prising that with more powerful induction discharges, and with 



* " Phil. Trans.," Part I, 1879. The Bakerian Lecture. 



2 K 2 



