432 
ME. W. CROOKES ON THE VISCOSITY 
THE ULTRA-GASEOUS STATE OF MATTER. 
710. A consideration of the curves of viscosity of the gases, especially hydrogen, 
which are given in the foregoing pages, will, I think, confirm the supposition that a 
gas, as the exhaustions become extreme, gradually loses its gaseous characteristics, 
and passes to what I have ventured to call an ultra-gaseous state. Certainly it ceases 
to possess many of the properties usually held to be the essential attributes of gaseity. 
For instance, Maxwell’s law that the viscosity of a gas is independent of pressure 
holds good to a certain point, and then it rapidly breaks down. All gases appear to 
obey Maxwell’s law between some limits of exhaustion, and diverge from it at 
others. The change to the ultra-gaseous state commences to be assumed at about an 
exhaustion of half a millim. In hydrogen the change then proceeds slowly, but in 
the other gases I have experimented with the change to ultra-gas takes place with 
greater rapidity. 
711. In gases, variation of pressure in different parts of a closed vessel equalises 
itself with great rapidity, but in the ultra-gaseous state differences of pressure may 
exist for twenty minutes or more in different parts of the apparatus. 
712. In gases, electrically charged bodies do not permanently retain their charge, 
but gradually discharge themselves. In ultra-gas, however, a pair of electrified gold 
leaves have remained repelled at absolutely the same angle for thirteen months.* 
713. Another property of gases is that of facilitating the cooling of bodies immersed 
in them, by communicating an increase of motion to the molecules of the gas which 
carry it to the walls of the containing vessel,— i.e., by carriage instead of convection. 
714. There is little difference in the rate of cooling with increased exhaustion, so 
long as we work with such ordinary good vacua as can be obtained by air-pumps. 
For if, on the one hand, there are fewer molecules impinging on the warm body (which 
is adverse to the carriage of heat), yet on the other hand, the mean length of path 
between collisions is increased so that the augmented motion is carried farther ; the 
number of steps by which the temperature passes from the warmer to the cooler body 
is diminished, but the value of each step is correspondingly increased. Hence the 
difference of velocity before and after impact may make up for the diminution in the 
number of molecules impinging. 
In gases, therefore, the rate of cooling is little affected by rarefaction, the law in 
this case being analogous to that governing the viscosity. 
715. In a paper which I have recently had the honour of reading before the Hoyal 
Society,! I show that when the exhaustion is carried to so high a point that the 
mean free path is comparable with the diameter of the containing vessel, the rate 
at which heat is conveyed across is much diminished. The molecules are now in 
the ultra-gaseous state, and further exhaustion produces a notable fall in the rate of 
* Pi’oc. Roy. Soc., No. 193, 1879, p. 347. 
t Ibid., No. 208, 1880, P . 239. 
