402 



NA TURE 



March 17, 1881 



ally merges, without break of continuity, into the radiant 

 streams of matter moving in the right directions to produce 

 gravity under Le Sage's sheltering principle, without the 

 necessity for adopting any of his postulates as to direction 

 of motion, or assuming a supply ' of matter from ultra- 

 mundane space in continuous currents (" ultramundane 

 corpuscles"). As this subject was carefully thought out 

 and dealt with by me in the Philosophical Magazine for 

 September and November, 1877, &c., I may perhaps 

 claim some right to say a few words about "radiant 

 matter.'' - 



The immense importance — in its possible practical 

 applications — of this remarkable self-correcting principle 

 (directly based on the mathematical results of the kinetic 

 theory) whereby particles of matter, left to their own 

 dynamics, rigidly adjust their motions so as to move in a 

 " radiant " manner [and to return energetically to this 

 beautifully symmetrical kind of motion when after dis- 

 turbance they are left to themselves], has, I venture to 

 think, not been duly appreciated. For, looking at the 

 case broadly, it would seem that this dynamical principle 

 is capable of affording a means for substantially satisfying 

 at least three fundamental objects in nature. For, firstlv, 

 it will appear evident that we can have thereby a means 

 perpetually present in every point of space for carrying 

 energy in a "radiant" manner {i.e. in the direction of the 

 rays of light from a point) in all possible directions. 

 Secondly, by this automatic system we can have a 

 mechanism capable of causing (under the sheltering prin- 

 ciple of Le Sage) the approach of the molecules of gross 

 matter at any point of space — such as exhibited in the 

 phenomena of "gravity" and (under modifying conditions 

 probably) the other phenomena of approach, "cohesion" 

 and "chemical action." Thirdly, since the "radiant" 

 character of the motion is inevitably attended by an exact 

 balance of the momenta at every point of space, we can 

 have in this system an exhaustless store of energy in 

 perfect eciuilibrium (and therefore concealed in its normal 

 state), competent to throw some rational light on such 

 unexplained phenomena as explosions, combustion, or the 

 violent developments of motion taking place in the 

 molecules of gross matter generally.^ 



As the phenomena of rarefied gases are attracting 

 attention at present, perhaps some calculations I have 

 made (based on the mathematical results of others) in 

 regard to conditions attending extreme rarefaction, may 

 not be without interest. The fact that the mean length 

 of path of the molecules (of a gas) increases in the triple 

 ratio of the mean distance on rarefying, leads to some 

 remarkable results, which would scarcely be expected 

 perhaps unless they had been worked out— and have their 

 application in regard to the long mean path required for 



• These postulates of Le Sage's theory relating to supply of matter from 

 boundless space, &c., were unfavourably criticised by the late Prof. Clerk 

 Maxwell (Encyc. Brit., 1875, under article "Atom"). Prof. Ma.vn-ell 

 fjnlffl's (p. 47) as follows:— •• We may observe that according to this theory 

 the habitable universe which we are accustomed to regard as the scene of a 

 magnificent illustration of the conservation of energy as the fundamental 

 principle of all nature, is in reality maintained in wori(ing order only by an 

 enormctis expenditure 'A external power, which would be nothing less than 

 rumous if the supply were drawn from anywhere else than from the infinitude 

 of space. 



It Will be seen that this objection vanishes by regarding the gravific aether 

 2S simply a slationary gas, within the limits of mean path of whose particles 

 the gravitating parts of the universe are immersed ; as then no supply of 

 matter or expenditure of external power is required. Also, it may be added, 

 that a. difficulty (mentioned p. 47 (jf same aiticle "Atom ") in regard to the 

 supposed exce.'Sive heating of gross matter that would occur under the 

 impacts of the gravific particles, was considered by the present writer i^Pliil. 

 Mag., November, 1877), and a means suggested for removing it without the 

 necessity for admitting any conditions which could be regarded as in them 

 selves improbable. 



^ It is said that Farad.iy was the first to use the expression "radiant 

 matter." 



3 To my mind, I must confess, it seems difficult to understand why 

 "potential " energy (in the sense of an energy which is not kinetic) appears 

 to be (comparatively speaking) so much brought to the fore-ground, to the 

 exclusion of the intelligible v'izvi of motion transferred from 7nattcr in 

 space. Is it not in general considered a right principle to give preference to 

 the intelligible or conceivable, in place of that which cannot appeal to our 

 reason? Evidently the term '■i-inelic" (.ipplied to energy) would be a re- 

 dundant and superfluous prefix, unless it were thereby implied that some 

 cM^r energy than "kinetic" energy, viz., anenergyzftM^Jw/wtJ/w/, existed. 



gravity. For it is a consequence of this that while the 

 mean distance of the molecules of a gas increases with 

 extreme slowness on rarefying, the mean path augments 

 at a great rate. 



This may be perhaps best elucidated by a mode of 

 illustration, which I have chosen with the endeavour, if 

 possible, to convey clear conceptions to the mind, which 

 is far more important than the mere writing down of 

 numbers (millions, &c.) which afford no defined idea at 

 all. Some conception of what actually occurs when a 

 gas is rarefied to a millionth of its normal density (a 

 common amount in experiments) may perhaps be pre- 

 sented to the mind by supposing a cubical box, say one 

 foot in the side, containing gas at normal density — 

 hydrogen for instance — to be opened in a room one 

 hundred feet in the side, containing a vacuum. This will 

 then accurately represent the actual degree of rarefaction 

 in the case under notice. The mean distance of the 

 molecules will then be increased (from known principles) 

 in the ratio of the linear side of the cubical box to that 

 of the cubical room, i.e. as I to loo. Since the mean 

 distance of the molecules at normal density is known to 

 have been about one seven-millionth of an inch ' (accord- 

 ing to the m.athematical results obtained by the late Prof. 

 Clerk Maxwell and others) ; the mean distance or rarefy- 

 ing to a millionth will become one seventy-thousandth of 

 an inch '>a hundred times greater, but still a very small dis- 

 tance). The mean length of path will have increased as the 

 t«^/i: contents of the room {i.e. in the triple ratio of the 

 mean distance). The mean length of path (which is known 

 to have been about -jnoou of an inch at normal density) 

 will now have rapidly risen to the very perceptible 

 dimensions of four inches (nearly). Here we have the 

 state of "radiant" matter (previously existing however 

 in the normal state of the gas, but concealed) coming to 

 be quite appreciable to the senses. For the gaseous 

 molecules now "radiate " regularly to a mean distance of 

 some inches from every point in the room ; and if a 

 portion of the gas were inclosed in a bulb, about four 

 inches in diameter, the molecules would (on the average) 

 strike across from one side to the other without colliding 

 among themselves : the beautiful '• radiant " character 

 of the motion then becoming lost, and the motion (and 

 consequent pressure) irregular, owing to the confined 

 space and absence of those mutual encounters among the 

 molecules by which the motion is forcibly corrected and 

 made symmetrical.'- It appears therefore that the truly 

 " radiant " character of the motion (if we use the word in 

 relation to the rays of light radiating from a luminous 

 point) would then cease — though no doubt the term 

 "radiant " may be also conveniently employed in another 

 sense, viz. to express the fact [when a portion of gaseous 

 matter is in a confined space where a proper adjustment 

 of pressure is not possible] that the molecules may, by 

 suitable means, be diverted from their paths, like the rays 

 of light, so as to move in a parallel (or common) direction, 

 and cast virtual shadows of objects placed in the bulb. 



It will be apparent therefore that the establishment of 



' I quote this dimension from a former paper, " On the Nature of what is 

 commonly called a 'Vacuum'" {P/iil. Mag.. August, 1877), a few of the 

 data of which it is convenient to use here as a commencement. It should be 

 remarked that Mr. Johnstone Stoney appears to have been the first to carry 

 out calculations regarding molecular dimensions and distances, and to deduce 

 therefrom conclusions regarding the number of molecules in unit of volume 

 of a so-called " vacuum " — which tended to upset preconceived ideas. 



^ It is evident that the " radiant " form of motion (or motion of the mole- 

 cules equally in all directions') is the sole condititn for equilibrium of pressure 

 in all directions in a gas, or for an exact balance of momenta in every direc- 

 tion. It is an obvious corollary from this — expressing a known fact — that if 

 any imaginary straight line be taken anywhere in a gas, as many molecules 

 at any instant are moving towards one extremity of the line as are moving 

 towards the opposite extremity — the resolved components of the motions 

 along the line being taken when the motions are oblique. It appears there- 

 fore that in order to bring gas rarefied to t.ne millionth under the normal 

 conditions for correcting the motions of its molecules (so as to move in the 

 normal "radiant" manner); it would be necessary to employ a containing 

 vessel of such size that the molecules can adjust their motions freely by 

 mutual encounters. Hence a containing vessel whose diameter was a con- 

 siderable multiple of the mean path (four inches in this case) would te 

 required — say some feet in diameter at least. 



