August 28, 1879] 



NATURE 



421 



'.he spectrum, having a wave-length 'of about 6S95. There is 

 a continuous spectrum beginning at about B, and a few fainter 

 lines beyond it, but they are so faint in comparison with this red 

 line that they may be neglected. This line is easily seen by 

 examining with a small pocket spectroscope the light reflected 

 from a good ruby. 



There is one particular degree of exhaustion more favourable 

 than any other for the development of the properties of radiant 

 matter which are now under examination. Roughly speaking, 

 it may be put at the millionth of an atmosphere. ^ At this degree 

 of exhaustion the phosphorescence is very strong, and after that 

 it begins to diminish until the spark refuses to pass,^ 



I have here a tube (Fig. 5) which will serve to illustrate the 



j'o millionth of an atmosphere 

 i3'5*739 millionths of an atmosphere 

 1,000,000* ,, ,, ,, 



Fig. 5. 



dependence of the phosphorescence oi the glass on the degree 

 of exhaustion. The two poles are at a and b, and at the end {c) 

 is a small supplementary tube, connected with the other by a 

 narro\y aperture, r»nd containing solid caustic potash. The tube 

 has been exhausted to a very high point, and the potash heated 

 so as to drive off moisture and injure the vacuum. Exhaustion 

 has then been recommenced, and the alternate heating and ex- 

 haustion repeated until the tube has been brought to the state in 

 wh'ch it now appears before you. When the induction-spark is 

 6rst turned on nothing is visible — the vacuum is so high that the 

 tube is non-conducting. I now warm the potash slightly, and 

 liberate a trace of aqueous vapour. Instantly conduction com- 

 mences, and the green phosphorescence flashes out along the 

 length of the tube. I continue the heat, so as to drive off more 

 gas from the potash. The green gets fainter, and now a wave 

 of cloudy luminosity sweeps over the tube, and stratifications 

 appear, which rapidly get narrower, until the spark passes along 

 the tube in the form of a narrow purple line. I take the lamp 

 away, and allow the potash ,to cool ; as it cools, the aqueous 

 vapour, which the heat had driven off, is re-absorbed. The 

 purple line broadens out, and breaks up into fine stratifications ; 

 these get wider, and travel towards the potash tube. Now a 

 wave of green light appears on the glass at the other end, sweep- 

 ing on and driving the last pale stratification into the potash ; 



= 0'00D76 minim. 



= i"omillim. 



^ 760 'o milliins. 

 ,1 M ,, „ =1 atmosphere. 



2 Nearly loo years ago Mr. Wm. Morgan communicated to the Royal 

 -^ >c.ety a paper entitled *' Klectrical Experiments made to ascertain the 

 ^.jn-conducting Power of a Perfect Vacuum, &c." I'he fullDwiiiK extracts 

 . r >m this paper, which was published in the Phil, Trans, for 1785 (vol, Ixxv. 

 p. 272), will be read with interest :— 



"A mercurial gage about 15 inches long, carefully and accurately boiled 

 till every particle of air was expelled from the inside, was coaled with tin-foil 

 5 inches down from its sealed end, and being inverted into mercury through 

 a perforation in the brass cap which covered the mouth of th-2 cistern ; the 

 whole w-as cemented together, and the air was exhausted from the inside of 

 the cistern thnjugli a valve in the brass cap, which, producing a perfect 

 vacuum in the gafie, formed an instrument peculiarly well adapted for ex- 

 periments^ of this kind. Things being thus adjusted (a small wire having 

 been previously fixed on the in,ide of the cistern t> form a communication 

 bi;t\veen the brass cap and the mercury, into which the gage was inverted) 

 t'le coated end was applied to the conductor of an electrical machine, and 

 : 'twithstanding every effort, neither the smallest ray of hght, nor the 

 'i^iitest chargi, c >uld ever be procured in this exhausted ga:jo. 

 "If the mercury in the gage be imperfectly boiled, the experiment will 

 not succeed ; but the colour of the electric light, which, in air rarefied by an 

 exhauster, is always violet or purple, appears in this case of a beautiful green, 

 and, what is very curious, the degree of the air's rarefaction may be nearly 

 determined by this means ; for I have known instances, during the course of 

 thcseexperim;nt;, where a small particle of air, 'having found its way into 

 the tube, the electric light became visible, and, as usual, of a green colour ; 

 but the charge being often repeated, the gage has at length cracked at its 

 sealed end, and in c msequence the external air, by being admitted into the 

 inside, has gradually produced a change in the electric liglit from green to 

 blue, from blue to indigo, and so on to violet and purple, till tlie medium has 

 at length become so dense as no longer to be a conductor of electricity. I 

 think there can be little doubt, from the above experiments, of the non-con- 

 ducting power of a perfect vacuum." 



'_' This seems to prove that there is a limit even in the rarefaction 'of air, 

 which sets bounds to its conducting power; or, in other words, that the 

 particles of air may be so far separated from each other as no longer to be 

 able to transmit the electric fluid : and if they are brought within a certain 

 distance of each other their conducting power begins, and continually increases 

 till their approach also arrives at its lunit." 



and now the tube glows over its whole length with the green 

 phosphorescence. I might keep it before you, and show the 

 green growing fainter and the vacuum becoming non-conducting, 

 but I should detain you too long, as time is required for the ab- 

 sorption of the last traces of vapour by the potash, and I must 

 pass on to the next subject. 



Radiant Matter proceeds in straight Lines 

 The radiant matter whose impact on the glass causes an evo- 

 lution of light, absolutely refuses to turn a corner. Here is a 

 V-shaped tube (Fig. 6), a pole being at each extremity. The 

 pole at the right side (a) being negative, you see that the whole 

 of the right arm is flooded with green light, but at the bottom it 

 stops sharply and will not turn the corner to get into the left 

 side. When I reverse the current and make the left pole nega- 

 tive, the green changes to the left side, always following the 

 negative pole and leaving the positive side with scarcely any 

 luminosity. 



In the ordinary phenomena exhibited by vacuum tubes — ^phe- 

 nomena with which we are all familiar — it is customary, in order 

 to bring out the striking contrasts of colour, to bend the tubes 

 into very elaborate designs. The luminosity caused by the phos- 

 phorescence of the residual gas follows all the convolutions into 

 which skilful glass-blowers can manage to twist the glass. The 



I'IG. 6. 



negative pole being at one end and the positive pole at the other, 

 the luminous phenomena seem to depend more on the positive 

 than on the negative at the ordinary exhaustion hitherto used to 

 get the best phenomena of vacuum tubes. But at a very high 

 exhaustion the phenomena noticed in ordinary vacuum tubes 

 when the induction spark passes through them — an appearance 

 of cloudy luminosity and of stratifications — disappear entirely. 

 No cloud or fog whatever is seen in the body of the tube, and 

 with such a vacuum as I am working with in these experiments, 

 the only light observed is that from the phosphorescent surface 

 of the glass. I have here two bulbs (Fig. 7), alike in shape and 

 position of poles, the only difference being that one is at an ex- 

 haustion equal to a few millimetres of mercury — such a moderate 

 exhaustion as will give the ordinary luminous phenomena — whilst 

 the other is exhausted to about the millionth of an atmosphere. 

 I will first connect the moderately exhausted bulb (A) with the 

 induction-coil, and retaining the pole at one side (o) always 

 negative, I will put the positive wire successively to the other 

 poles with which the bulb is furnished. You see that as I 

 change the position of the positive pole, the line of violet light 

 joining the two poles changes, the electric current always choosing 

 the shortest path between the two poles, and moving about the 

 bulb as I alter the position of the wires. 



Tliis, then, is the kind of phenomenon we get in ordinary 

 exhaustions. I will now try the same experiment with a bulb (b) 



