56 



NA TURE 



[NoVliMBER 112, 1908 



The electropositive metals used for the kathode were: — 

 (1) the liquid alloy of sodium and potassium, which was 

 smeared over the kathode, and (2) calcium, a thin plate 

 of which was affixed to the front of the kathode. With 

 these kathodes, the pressure in the tube could be reduced 

 to very low values without making the discharge so 

 difficult as to lead to perforation of the tube by sparking, 

 and accurate measurements of the position of the patches 

 of phosphorescence could be obtained at leisure. : 



■ The results obtained at these low- pressures are very 

 interesting. Whatever kind of gas may be used to fill 

 the tube, or whatever the nature of the electrode, the de- 

 flected phosphorescence splits up into two patches. For 

 one of these patches the maximum value of elm is about 

 10', the value for the hydrogen atom ; while the value for 

 the other patch is about 5x10', the value for a particles 

 or the hydrogen molecule. Examples of the appearance 

 of this phosphorescence are given in Figs. 10, 11, and 12. 

 In Fig. 12 the magnetic force was reversed. 



Hydrogen Helium -Air 



The differences in the appearance arc due to differences 

 in the pressure rather than to differences in the gas ; 

 for at slightly higher pressures than that corresponding to 

 Fig. 12, the appearance shown in Figs. 10 and 11 can be 

 obtained in air. In all these cases the more deflected patch 

 corresponds to a value of about 10* for ejm, while cjin for 

 tlie less deflected patch is about 5x10°. 



It will be noticed that in Fig. 11 there is no trace in 

 the helium tube of rays for which e/ni = 2-5 x 10^, which 

 uere found in helium tubes at higher pressures; at inter- 

 mediate pressures there are three distinct patches of helium, 

 fur the first of which c/m= 10*, for the second 

 I- m=5xio^, and for the third e/7n = 2-5Xio' approxi- 

 mately. Helium is a case where there are characteristic 

 rays — i.e. rays for which c/m=io''/M, where M is the 

 atomic weight of the gas, when the discharge potential 

 is comparatively small, and not when, as at very low 

 pressures, the discharge potential is very large. I think 

 it very probable that, if we could produce the positive rays 

 with much smaller potential differences than those used in 

 these experiments, we might get the characteristic rays 

 for other gases. I ain at present investigating with this 

 object the positive rays produced when the perforated 

 kathode is, as in Wehnelt's inethod, coated with lime, 

 when a potential difference of 100 volts or less is able 

 to produce positive rays. The interest of the experiments 

 at very low pressures lies in the fact that in this case the 

 rays are the same whatever gas may be used to fill the 

 tube ; the characteristic rays of the gas disappear, and 

 we get the same kind of carriers for all substances. 



I would especially direct attention to the simplicity of 

 the effects produced at these low pressures ; only two 

 patches of phosphorescence are visible. This is, I think, 

 an important matter in connection with the interpretation 

 of these results ; for at these low pressures we have to 

 deal, not only with the gas with which the tube was 

 originally filled, but also with the gas which is given off 

 by the electrodes and the walls of the tube during the 

 discharge ; and it might be urged that at the.se low 

 pressures the tube contained nothing but hydrogen given 

 out by the electrodes. I do not think this explanation is 

 feasible, for the following reasons : — 



(i) The gas developed during the discharge is not whollv 

 hydrogen; if the discharge is kept passing long enough 

 to develop so much gas that the discharge through the 

 gas is sufficiently luminous to be observed bv a spectro- 

 scope, the spectrum always .showed, in addition to the 

 hydrogen lines, the nitrogen bands ; indeed, the latter 



NO. 2037, VOL. 79] 



were. generally the most conspicuous part of the spectrun.. 

 If the phosphorescent . screen on . which, the positive rays 

 impinge. is observed during- the .time this is being given 

 off, the changes which take place in the appearance of 

 the screen are as follows: — If, to begin with, the pressure 

 is so slow that the phosphorescent patches are reduced to 

 two -bright spots, then, as -the pressure begins to go up 

 owing tO'- the evolution of the gas, the deflection of the 

 spots • increases. This • is owing to the reduction in the 

 velocity of the rays consequent upon the reduction of the 

 potential difference between the terminals of the tube, as 

 at this stage an increase in the. pressure facilitates the 

 passage of the discharge.- In addition to the increase in 

 the displacement there is an increase in the area of the 

 spots- giving a greater range of values of c/m; this is 

 owing to the increase in the number of collisions made 

 by the particles in the rays on their ■ way to the screen. 

 .As more and more gas is - evolved the patches get larger, 

 and finally overlap ; the existence of the second patch being 

 indicated by a diminution in the brightness of the phos- 

 phorescence at places outside its boundary. As the pressure 

 increases the luminosity gets more and more continuous, 

 and - we finally get to the continuous band, as shown in 

 Fig. 6. At this stage it is probable that there may be 

 enough luminosity to give a spectrum showing the nitrogen 

 lines, indicating that a considerable part of the gas in the 

 tube is air. It is especially to be noted that during this 

 pi'ocess, ■ when gas was coming into the tube, there has 

 been no development of patches in the phosphorescence 

 indicating the presence of new rays ; on the contrary, one 

 type of carrier — that corresponding to e/m = 5Xio" — has 

 disappeared. The presence of the nitrogen bands in the 

 spectrum shows that nitrogen is carrying part of the dis- 

 charge, and yet there are no rays characteristic of nitrogen 

 to be observed on the screen, a proof, it seems to me, 

 that different gases may be made by strong electric fields 

 to give off the same kind of carriers of positive electricity. 



Another result, which shows that the positive rays are 

 the same although the gases are different, is the following. 

 The tube was pumped until the pressure was much too 

 low for the discharge to pass, then small quantities of 

 the following gases were put into the tube : — air, carbonic 

 oxide, hydrogen, helium, neon (for which I am indebted 

 to the kindness of .Sir James Dewar) ; the quantity admitted 

 was adjusted so that it was sufficient to cause the discharge 

 to pass, and yet did not raise the pressure beyond the point 

 where the phosphorescence is discontinuous. In every case 

 there were patches corresponding to c/"i = io*, c/m = 5Xio^, 

 and except with helium these were the only patches; in 

 helium, in addition to the two already mentioned, there 

 was a third patch, for w-hich c/m =2-5 X 10'. 



I also tried another method of ensuring that at these 

 low pressures there were other gases besides hydrogen in 

 the tube. I filled the tube with helium, and after exhaust- 

 ing to a fairly low pressure by means of the mercury 

 pump, I performed the last stages of the exhaustion by 

 means of charcoal cooled with liquid air. This charcoal 

 absorbs very little helium in comparison with other gases, 

 so that it is certain that there was helium in the tube. 

 The appearance of the phosphorescent screen of tubes 

 exhausted in this way did not differ from those exhausted 

 solely by the pump. 



The most obvious explanation of these effects seems to 

 me to be that under very intense electric fields different 

 substances give out particles charged with positive elec- 

 tricity, and that these particles are independent of the 

 nature of the gas from which they originate. These 

 particles are, so far as we know at present, of two kinds ; 

 for one kind e/uihas the value of lo', that of an atom 

 of hydrogen; for the other kind em has half this value, 

 I.e. it has the same value as for the a. particles from radio- 

 active substances. 



This agreement in the maximum value of ehn at 

 different pressures is a proof that this is a true maximum, 

 and that there are not other more deflected rays not strong 

 enough to produce visible phosphorescence ; for if this were 

 the case — i.e. if the value of elm for a particle that had 

 never lost its charge tcmporarilv bv collision were greater 

 than 10* — we should expect to get larger values for elm 

 at low pressures than at high. 



