Augusi 28, 1879] 



NATURE 



419 



-pc 



ON RADIANT MA TTER 1 



""O throw light on the title of this lecture I must go back more 

 than sixty years — to 1816. Faraday, then a mere student 

 and ardent experimentalist, was twenty-four years old, and at 

 this early period of his career he delivered a series of lectures 

 on the general properties of matter, and one of them bore the 

 remarkable title, "On Radiant Matter." The great philosopher's 

 notes of this lecture are to be found in Dr. Bence Jones's " Life 

 and Letters of Faraday," and I will here quote a passage in 

 which he first employs the expression Radiant Matter : — 



"If we conceive a change as far beyond vaporisation as that 

 is above fluidity, and then take into account also the proportional 

 increased extent of alteration as the changes rise, we shall per- 

 haps, if we can form any conception at all, not fall far short of 

 radiant matter ; and as in the last conversion many qualities were 

 lost, so here also many more would disappear." 



Faraday was evidently engrossed with this far-reaching specu- 

 lation, for three years later — in 1819 — we find him bringing 

 fresh evidence and argument to strengthen his startling hypo- 

 thesis. His notes are now more extended, and they show that 

 in the intervening three years he had thought much and deeply 

 on this higher form of matter. He first points out that matter 

 may be classed into four states — solid, liquid, gaseous, and 

 radiant — these modifications depending upon differences in their 

 several essential properties. He admits that the existence of 

 radiant matter is as yet unproved, and then proceeds, in a series 

 of ingenious analogical arguments, to show the probability of its 

 existence. ' 



If, in the beginning of this century, we had asked, What is a 

 gas ? the answer then would have been that it is matter, ex- 

 panded and rarefied to such an extent as to be impalpable, save 

 when set in violent motion ; invisible, incapable of assuming or 

 of being reduced into any definite form like solids, or of form- 

 ing drops like liquids ; always ready to expand where no resist- 

 .ance is offered, and to contract on being subjected to pressure. 

 Sixty years ago juch were the chief attributes assigned to gases. 

 Modem research, however, has greatly enlarged and modified 

 our views on the constitution of these elastic fluids. Gases are 

 now considered to be composed of an almost infinite number 

 of small particles or molecules, which are constantly mov- 

 ing in every direction with velocities of all conceivable mag- 

 nitudes. As the-i^e molecules are exceedingly numerous, it fol- 

 lows that no molecule can move far in any direction without 

 coming in contact with some other molecule. But if we exhaust 

 the air or gas contained in a closed vessel, the number of mole- 

 cules becomes diminished, and the distance through which any 

 one of them can move without coming in contact with another is 

 increased, the length of the mean free path being inversely pro- 

 portional to the number of molecules present. The further this 

 process is carried the longer becomes the average distance a 

 molecule can travel before entering into collision ; or, in other 

 words, the longer its mean free path the more the physical pro- 

 perties of the gas or air are modified. Thus, at a certain point, 

 the phenomena of the radiometer become possible, and on push- 

 ing the rarefaction still further, i.e., decreasing the number of 



' A lecture delivered to the British Associ.ation for the Advancement of 

 Science, at Sheffield, Friday, August 22, 1879, by William Crookes. F.R.S. 

 _ ' "1 may now notice a curious jirogression in physical properties accompany- 

 ing changes of form, and which is perhaps sufficient to induce, in the inven- 

 tive and sanguine philosopher, a considerable degree of belief in the asso- 

 ciation of the radiant form with the others ia the set of ch.anges I have 

 mentioned. 



" As we ascend from the solid to the fluid and gaseous st.ates, physical 

 properties diminish in number and variety, each stale losing some of those 

 wWch belonged to the preceding state. When solids are converted into 

 fluids, all the varieties of hardness and softness are necess.-irily lost. Crys- 

 talline and other shapes are destroyed. Opacity and colour frequently give 

 way to a colourless transparency, and a general mobility of particles is 



"Passing onward to the gaseous state, still more of the evident characters 

 c^f bodies are annihilated. The immense difl-erences in their weight almost 

 oisappear ; the remains of dilTerence in colour that were left are lost. Trans- 

 parency becomes universal and they are all elastic. 'Ihey now form but 

 one set of substances, and the varieties of density, hardness, opacity, colour, 

 elasticity, and form, which render the number of solids and fluids almost 

 infinite, are now supplied by a few slight variations in weight, and lome 

 unimportant shades of colour. 



;• To those, therefore, who admit the radiant form of matter, no difficulty 

 exisu in the simplicity of the properties it possesses, but rather an argument 

 in their favour. These persons show you a gradual resignation of properties 

 in the matter we c«n appreciate as the matter ascends in the scale of forms 

 ami they would be supnsed if that effect were to cease at the gaseous ilate! 

 They point out the greater exertions which nature makes at each step "of 

 the change, and think that, consistently, it ought to be greatest in the pas- 

 sage from the gaseous to the radiant form."— ZrV* ariti Litters of Faradav 

 vol. i. p. 308, ' 



molecules in a given space and lengthening their mean free path, 

 the experimental results are obtainable to which I am now about 

 to call your attention. So distinct are these phenomena from 

 anything which occurs in air or gas at the ordinary tension, that 

 we are led to assume that we are here brought face to face with 

 matter in a fourth state or condition, a condition as far removed 

 from the state of gas as a gas is from a liquid. 



Mean Fre; Path. Radiant Matter 

 I have long believed that a well-known appearance observed 

 in vacuum tubes is closely related to the phenomena of the mean 

 free path of the molecules. When the negative pole is examined 

 while the discharge from an induction-coil is passing through an 

 exhausted tube, a dark space is seen to surround it. This dark 

 space is found to increase and diminish as the vacuum is varied, 

 in the same way that the mean free path of the molecules 

 lengthens and contracts. As the one is perceived by the mind's 

 eye to get greater, so the other is seen by the bodily eye to in- 

 crease in size ; and if the vacuum is insufficient to permit mucS 

 play of the molecules before they enter into collision, the pas- 

 sage of electricity shows that the " dark space " has shrunk to 

 small dimensions. We naturally infer that the dark space is the 

 mean free path of the molecules of the residual gas, an inference 

 confirmed by experiment 



I will endeavour to render this " dark space " visible to all 

 present. Here is a tube (Fig. i) having a pole in the centre in 



the form of a metal disk, and other poles at each end. The 

 centre pole is made negative, and the two end poles connected 

 together are made the positive terminal. The dark space will be 

 in the centre. When the exhaustion is not very great the dark 

 space extends only a little on each side of the negative pole in 

 the centre. When the exhaustion is good, as in the tube before 

 you, and I turn on the coil, the dark space is seen to extend for 

 about an inch on each side of the pole. 



Here, then, we see the induction spark actually illuminating 

 the lines of molecular pressure caused by the excitement of the- 

 neg.ative pole. The thickness of this dark space is the measure 

 of the mean free path between successive collisions of the 

 molecules of the residual gas. The extra velocity with which the 

 negatively electrified molecules rebound from the excited pole, 

 keeps back the more slowly moving molecules which are 

 advancing towards that pole. A conflict occurs at the boundary 

 of the dark space, where the luminous margin bears witness to 

 the energy of the discharge. 



Therefore the residual gas — or, as I prefer to call it, the 

 gaseous residue — within the dark space, is in an entirely different 

 state to that of the residual gas in vessels at a lower degree of 

 exhaustion. To quote the words of our last year's President, in 

 his address at Dublin : — 



' ' In the exhausted column we have a vehicle for electricity not 

 constant like an ordinary conductor, but itself modified by the 

 passage of the discharge, and perhaps subject to laws differing 

 materially from those which it obeys at atmospheric pressure." 



In the vessels with the lower degree of exhaustion, the length 

 of the mean free path of the molecules is exceedingly small as 

 compared with the dimensions of the bulb, and the properties 

 belonging to the ordinary gaseous state of matter, depending upon 

 constant collisions, can be observed. But in the phenomena 

 now about to be examined, so high is the exhaustion carried that 

 the dark space around the negative pole has widened out till it 

 entirely fills the tube. By great rarefaction the mean free path 



