March lo, 1881] 



NATURE 



445 



givea in the foregoing pages, confirms the supposition that a 

 ga':, as the exhaustions become extreme, gradually loses its 

 gaseous characteristics, and pa ses to what tlie author has ven- 

 tured 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 inde- 

 pendent 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. 

 Thus the nearly perfect gas hydrogen shows signs of increasing 

 in viscoiity as the pressure approaches 760 millims., and it is 

 very improbable that its viscosity would remain the same if the 

 pressure were to be considerably increased. Between 5 and 35 

 millims. the respective viscosities of carbonic anhydride, car- 

 bonic oxide, nitrogen, oxygen, and air scarcely vary at all, show- 

 ing that between these limits they are practically as "perfect" 

 gases as hydrogen is throughout the whole barometric range from 

 760 millims. to i millim., and here therefore they obey Maxwell's 

 law as perfectly as hydrogen does. The change to the ultra- 

 gaseous state commences to be assumed at about an exhaustion 

 of half a millim. In hydrogen the change tlien proceeds 

 slowly, but in the less perfect gases experimented with, the 

 change to ultra gas takes place with greater rapidity. 



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. 



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 re- 

 mained repelled at absolutely the samr angle for thirteen 

 m onths.i 



Another property of gases is that of facilitating the cooling of 

 bodies immer>ed 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 carnage instead of convection. 

 There is little difference in the rate of cooling \\ith increased ex- 

 haustion, 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 

 averse 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 in- 

 creased. Hence the difference of velocity before and after 

 impact may make up for the diminution in the number of mole- 

 cules impinging. 



In gases, therefore, the rate of cooling is little affected by rare- 

 faction, the law in this case being analogous to that governing 

 the viscosity. 



In a paper which the author has recently read before the Royal 

 Societj',^ he shows 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 cooling, an increase of exhaustion from 20 M to 2 M retarding 

 the carriage of heat more than all the previous exhaustion from 

 760 millims. to 20 M. 



The author has shown elsewhere 3 that the property of gaseity 

 is pre-eminently a property dependent on collisions. A given 

 space full of air at the ordinary pressure contains millions of 

 millions of molecules rapidly nijving in all directions, each mole- 

 cule momentarily encountering millions of other molecules in a 

 second. In such a case the lengt'a of the mean free path of the 

 molecules is exceedingly small compared with the dimensions of 

 the containing vessel, and those properties are observed which 

 constitute the ordinary gaseous state of matter — properties which 

 depend upon constant collisions. 



The gaseous state continues so long as the collisions are 

 almost infinite in number, and of inconceivable irregularity. 

 But in such high vacua as are now described the free path of the 

 molecules is made so long that the hits in a given time may be 

 disregarded in comparison to the misses, and the average mole- 

 cule is allowed to obey its own motions or laws without inter- 

 ference ; and when the mean free path is comparable to the 



' Proceedings of the R. S. , No. 193, 1879, P- 347- 



2 Proc. R.'b.. No. 2o8, 1880, p. 239. 



3 Proc. R. S., No. 205, 1880, p 469. 



dimensions of the containing vessel, the properties which con- 

 stitute gaseity are reduced to a minimum, and the matter then 

 becomes exalted to an ultra-gaseous state. 



In the ultra-gaseous state properties of matter which exist 

 even in the gaseous state are shown directly, whereas in the 

 state of gas they are only shown indirectly, by viscosity and so 

 forth. 



The ordinary laws of gases are a simplification of the effects 

 arising from the propeities of matter in the idtra-gaseous state ; 

 such a simplification is only permissible when the mean length of 

 path is small compared with the dimensions of the vessel. For 

 the sake of simplicity we make abstraction of the individual 

 molecules, and feign to our imagination conliinwiis matter of 

 which the fundamental properties — such as pressure varying as 

 the density, and so forth — are ascertained by experiment. A gas 

 is nothing more than an assemblage of molecules conlem|ilated 

 from a simplified point of view. When we deal with phenomena 

 in which we are obliged to individually contemplate molecules, 

 we must not speak of the assemblage as gas. 



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 nltra-gas with its associated phenomena is at the 

 maximum at a millionth of an atmosphere. Here the mean free 

 path is about 4 inches long, sufficient to strike across the 

 exhausted tube. But it has been shown by many expeiimentalists 

 that at exhaustions so low that the contents of the tube are 

 certainly not in the ultra-gaseous state, the phenomena of phos- 

 phorescence 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 pro- 

 ducing green phosphorescence can be projected 102 millimetres 

 from the negative pole when the pressure is as high as o'324 

 millim. or 427 M. In this case the mean free path of the mole- 

 cules is 0*23 millim. ; and it is not surprising that with more 

 powerful induction discharges, and with special appliances for 

 exalting the faint action to be detected, the above-named pheno- 

 mena can be produced at still higher pressures. 



It must be remembered that we know nothing of the absolute 

 length of the free path or the absolute velocity of a molecule ; 

 these may vary almost from zero to infinity. We must limit 

 ourselves to the mean free path and the mean velocity, and all 

 that these experiments show is that a few molecules can ti'avel 

 more than a iiundred times the mean free path, and with perhaps 

 a correspondmg increase over the mean velocity, befo.e they are 

 stopped by collisions. With weak electrical power the special 

 phosphorogenic action of these few molecules is too faint to be 

 noticed ; but by intensifying the discharge the act'on of the 

 molecules can be so increased as to render their presence visible. 

 It is al-o probable that the absolute velocity of the m ilecules is 

 increased so as to make the mean velocity with which they leave 

 the negative pole greater than that of ordinary gaseous molecules. 

 This being the case, they w ill not easily be stopped or deflected 

 by collisions, but will drive through obstacles and so travel to a 

 greater distance. 



If this view is correct, it does not follow that gas and ultra 

 gas can co-exist in the same vessel. All that can be legitimately 

 inferred is, that the tw'O states insensibly merge one into the 

 other, so that at an intermediate point we can by appropriate 

 means exalt either the phenomena due to gas or to ultra gas. 

 The same thing occjrs between the .states of solid and liquid 

 and liquid and gas. Tresca's experiments on the flow of solids 

 prove that lead and even iron, at the common temperature, 

 possess properties which strictly appertain to liquids, whilst 

 Andrews has shown that liquid and gas may be made to merge 

 gradually one into the other, so that at an intermediate point the 

 substance partakes of the properties of both states. 



Note on the Reduction of Mr. Crookes's Experiments on the 



Decrement of the Arc of Vibration of a Mica Plate oscillating 



within a Bulb containing more or less Rarefied Gas ^ 



The determination of the motion of the gas within the bulb, 



which would theoretically lead to a determination of the o 



efficient of viscosity of the gas, forms a mathematical problem 



' Pkt't. Trans, part i, 1879, the Bakeriatl Lecture. 

 ^ Abstract of a paper read before the Royal So 

 Prof. G. G. Stokes, Sec.R.S. 



ety, February 17, by 



