646 



NATURE 



[February 13, 1913 



were tried (the experimenting chamber) was sealed on 

 to the testing- vessel, but separated from it by a tap. 

 Thus the pressure in the experimenting chamber was 

 not restricted to being the same as that in the testing 

 vessel, but might have the value which seemed most 

 appropriate for any particular type of experiment. 

 After these experiments were over, the tap was turned 

 and some of the gases from the experimenting cham- 

 ber let into the testing vessel ; a photograph was 

 then_ taken, and by comparing it with one taken before 

 turning the tap the new gases present in the e.xperi- 

 ment chamber could be detected. The processes by 

 which I have hitherto got the most plentiful supply of 

 X, are : — 



(i) By bombarding with kathode rays metals and 

 other bodies. 



(2) By the discharge from a Wehnelt kathode 

 through a gas at a low pressure. 



(3) By an arc discharge in a gas at a comparatively 

 high pressure. 



By far the larger number of the experiments were 

 made by bombarding metals, but I will begin bv 

 describing an experiment with the arc, as it raises the 

 question of the origin of these lines in a very direct 

 way. An arc between iron wires passed through 

 hydrogen at about 3 cm. pressure (in this case all the 

 kathode rays would be absorbed quite close to the 

 electrode) for an hour or so, and the gases liberated 

 in the experimenting chamber tested; X,, helium, and 

 neon were found. The experiment, using the same 

 wires for terminals, was repeated the next day; the 

 three gases were again found. On the ne.xt day, still 

 using the same wires, the arc was passed through 

 oxygen; the X, line was still there, though much 

 fainter than before ; the helium and neon could not be 

 detected with certainty. The next day, using the 

 same terminals, the arc was again passed through 

 oxygen ; not one of the lines could be detected. This 

 looks as if these substances were produced by the 

 arc passing through hydrogen. It was found, how- 

 ever, that, still keeping to the same terminals, on 

 pumping the oxygen carefully out and filling up again 

 with hydrogen, the arc through the hydrogen now did 

 not give even a trace of these lines. On replacing the 

 old iron wires by new ones, and sending the arc 

 through the hydrogen, the lines reappeared. This 

 experiment seems to me to point very clearly to the 

 conclusion that these gases were in the terniinals to 

 begin with, were removed from them by the long- 

 continued sparking, and were not produced de novo 

 by the arc. 



In the experiments when the discharge was pro- 

 duced in a tube with a Wehnelt kathode, the potential 

 difference between the terminals was only 220 volts, 

 so that the kathode rays in the tube had only a frac- 

 tion of the energy they had when the discharge was 

 produced by an induction coil ; X, and helium ap- 

 peared when the discharge passed through this tube. 

 I did not detect any neon. 



The method which gave X, and also the other 

 gases, in the greatest abundance, was to bombard 

 metals, or indeed almost any substance, with kathode 

 rays. The tube used for this purpose had a curved 

 kathode, which focussed the rays on a table on which 

 the substance to be bombarded was placed. The sub- 

 stance, round the spot struck by the rays, was gener- 

 ally raised to a bright red heat by the bombardment; 

 the bombardment was as a rule continued for five or 

 six hours at a time. I have got the X, line, as a 

 rule, accompanied at first bv the helium line, and 

 somewhat less frequently by the neon line, when these 

 following substances (which include nearly all I have 

 tried) were bombarded : iron, nickel, oxide of nickel, 

 zinc, copper, various samples of lead, platinum, two 



NO. 2259, VOL. 90] 



meteorites, and a specimen of black mica given me 

 by Sir James Dewar, which was remarkable for the 

 amount of neon it gave off. 



The most abundant supply of X3 came from 

 platinum, and I will describe an experiment with this 

 metal. A piece of platinum foil was bombarded on 

 four days, and the gases produced each day examined. 

 At the end of the first day's bombardment it was 

 found that the line due to X3 was very strong, those 

 due to helium and neon weaker, but still quite con- 

 spicuous. The gases produced the first day were well 

 washed out of the tube, and the foil bombarded for a 

 second day. The gases formed proved to be much 

 the same as on the first day ; there was no appre- 

 ciable diminution. The examination of the result of 

 the third day's bombardment showed that the X3 line 

 had diminished considerably, the lines due to helium 

 and neon perceptibly. When the gases produced on 

 the fourth day's bombardment were examined it was 

 found that the X, and helium had diminished to such 

 an extent that the lines were barely visible. I could 

 not see the neon line at all. In this case the helium 

 was not eliminated until the fourth day. In general 



1 have found that the helium disappeared long before 

 the X, gas. Thus a piece of old lead I bombarded 

 gave off appreciable quantities of helium from the 

 first dav's bombardment, very little on the second day, 

 and none that I could detect on the third or subse- 

 quent days. The X,, on the other hand, came off in 

 considerable quantities up to the end of the experi- 

 ment, which lasted for six days. I attribute the 

 superior elimination of X, in the case of the platinum 

 foil to the fact that during the whole time the bom- 

 bardment was concentrated on a patch only about 



2 mm. in diameter, while the lead melted under the 

 bombardment, so that fresh portions were continually 

 being exposed to the rays. A piece of Kahlbaum's 

 chemically pure lead gave appreciable amounts of X, 

 and helium, tliough not nearly so much as the old 

 lead. I tried some lead which had just been precipi- 

 tated, but could not detect either Xj or helium. 



In the course of the experiments with old lead I 

 let hydrogen into the experimenting chamber to see 

 if it would increase the amount of X,, but could not 

 detect any effect. On one occasion I let in oxygen 

 when nickel was bombarded, also without any appre- 

 ciable effect. I think these experiments are in favour 

 of the view that these gases are present in the metal 

 independently of the bombardment, and are liberated 

 bv the action of the kathode rays. They are sur- 

 prisingly firmly held by the metal, and cannot, so 

 far as my experience goes, be got rid of by heating. 

 I kept a piece of lead in a quartz tube boiling in a 

 vacuum for three or four hours, until all but a 

 quarter of the lead had boiled away, and examined 

 the gases given off during this process ; neither X, 

 nor helium could be detected. I then took the quarter 

 that remained and bombarded it, and got appreciable 

 amounts of X, and helium. On a second bombard- 

 ment the X, was visible but the helium had dis- 

 appeared. As an instance of the way these gases can 

 stick to metals even when in solution or chemical 

 combination, I may mention that though, as I have 

 said, platinum foil after long exposure to kathode rays 

 is freed from these gases, vet I got appreciable quan- 

 tities of X, and helium, though no neon from plati- 

 num sponge freshly prepared from platinic chloride. 



The reason helium is obtained by heating the glass 

 of old Rontgen-ray bulbs is, I think, that after libera- 

 tion by the kathode rays, the helium either adheres 

 to the surface or is absorbed in a much looser way 

 than before it was liberated. The question as to how 

 these gases get into the metals is a most interesting 

 one; are they absorbed in the process of manufacture? 



