3i6 



NA TURE 



[August 2, 1906 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of 'Nature. 

 No notice is taken of anonymous communications.] 



The Positive Charge carried by the a Particle. 



Some time ago I made the suggestion in Nature (March 9, 

 1905) that the a particle was initially uncharged on expul- 

 sion, and that it gained its charge subsequently by col- 

 lision with atoms in its path. I need only now repeat 

 that the suggestion was based on the brilliant work o 

 Bragg in Australia, who showed that the a particle passes 

 through, rather than collides with, the atoms of solid or 

 gaseous matter in its path, and that whether uncharged 

 or not initially, it must, equally with the atom struck, 

 become charged positively after the encounter by the de- 

 tachment of a negative electron. 



Recently P. Ewers {Physikalische Zeitschrift, March i), 

 using the a particles from polonium, attempted to put the 

 view to. an experimental test with negative results, and 

 concluded against the probability of the hypothesis. Bragg 

 (Phys. Zeit., July i) has pointed out that Ewers's experi- 

 ments by no means settle the question, and, indeed, he 

 evidently considers it a question which cannot be settled 

 experimentally. Certainly the requisite conditions to be 

 fulfilled for a positive result are so rigorous that no one 

 could be certain they had been fulfilled, and it is impos- 

 sible to disprove the view by a negative result. 

 But it is obvious that a positive result, that is, 

 the actual isolation of the a particle in an un- 

 charged state, would settle the question. This I have 

 been fortunate enough to do, although only after a long 

 experience of negative results where it might reasonably 

 have been concluded the requisite conditions had been 

 realised. "The best laid schemes of mice and men gang 

 aft a-gley." A deterEiiining factor in the problem con- 

 ditioning whether a positive or negative result is obtained 

 could not possibly have been foreseen, and it was only when 

 all hope of getting anything but a negative result had 

 been abandoned, and what was intended to be a final 

 experiment was being performed, that a slight change in 

 one of the factors happened to eliminate the disturbing 

 cause, and I obtained the coveted positive result. The 

 precise nature of this disturbing influence is, perhaps, not 

 yet fully demonstrated, although personally I think I now 

 hold the clue. But there is not the slightest doubt that 

 the o particle initially expelled is not charged as the ex- 

 periments given prove. 



The essential conditions are two. In the first place, the 

 a particle must be examined in a vacuum such that during 

 its path it does not encounter a single gas molecule. 

 Secondly, the layer of radio-active matter from which it is 

 e.xpelled must not be more than one molecule thick, and 

 must not be mixed with or overlaid by inactive matter. 

 These conditions being fulfilled, the a particle will not 

 traverse a single atom after expulsion, and if uncharged 

 initially must remain so. As a third condition, it is de- 

 sirable that the test for the charge shall be made on the 

 particle during its flight. It is at least conceivable that 

 an uncharged a particle striking a plate will convey to it a 

 positive charge if the electron detached from the uncharged 

 a particle on impact has suRicient energy to escape the 

 plate. 



The second condition is, as may be imagined, the 

 difficult one to make sure of. I hoped to secure it by 

 using radium C as the source of the rays. The rate of 

 its disintegration is so rapid that there is only just the 

 necessary time for an experiment to be carried out. Hence 

 the actual number of atoms of the radio-active substance 

 is for radium C the minimum it is possible to employ. 

 Moreover, this number can readily be calculated, and since 

 it is deposited from a gas uniformly on the exposed surface, 

 not only can an experiment be devised so that the thickness 

 of the deposited layer fulfils the monomolecular condition, 

 but, what is equally important, it can be assumed with 

 reasonable certainty that the radio-active layer is not 

 overlaid or mixed with inactive matter. 



NO. 191 8, VOL. 74] 



With regard to the first condition, all the factors are- 

 known, and the necessary conditions can readily be cal- 

 culated by two independent methods, which, as it proved, 

 are strikingly verified by the actual results obtained. The 

 only pitfall is in the altogether exaggerated impression 

 which is abroad as to the ease with which a high degree 

 of vacuum can be obtained by modern methods. 



The third condition was realised by using the magnetic 

 deviation of the o rays as a test for their charge. The- 

 rays passed out of the capillary tube from a deposit of 

 radium C at the far end. This was obtained by the use 

 of the emanation from 30 mg. of radium. Conditions were 

 arranged so that the rays were completely deviated under 

 ordinary conditions, and with thf magnetic field on did not 

 succeed in escaping from the tube, and the experiment con- 

 sisted simply in re-examining the deviation in the highest 

 vacuum that could be produced. 



Long series of negative results led to the refinement of 

 each essential condition until it seemed no further improve- 

 ment was possible, and a wide margin of probability that 

 the essential conditions had been realised had been secured. 

 A most unmistakable negative result was obtained. But 

 the next experiment intended to confirm this finally was 

 as unmistakable a positive result as the other had been a 

 negative one. In a partial vacuum the rays were completely- 

 deviated. In the highest vacuum the field made no per- 

 ceptible difference. Between the two experiments there 

 were two slight differences of conditions : (i) In the second 

 experiment the radio-active deposit had been heated irt 

 vacuo after removal of the emanation and disappearance 

 of radium A in order to remove a possible overlying film 

 of condensed gas. (2) In the first experiment the emanation 

 had been left in the capillary 2 hours 25 minutes, in 

 the second i hour 30 minutes, the volume occupied by the 

 emanation being less in the latter case. 



In a third experiment the heating of the radio-active 

 surface was omitted, and the emanation was allowed to 

 act for only 45 minutes. The result was unequivocally 

 positive. 



In a fourth experiment the film was heated, and the 

 emanation left in i hour 20 minutes, reproducing prac- 

 tically the conditions of the second experiment. Again the 

 result was positive, and the magnetic field produced no 

 appreciable effect in a high vacuum. But this experiment 

 was continued for nearly two hours after the start, and at 

 the end of the time the radiation, although, of course, 

 much enfeebled, was quite intense enough for the purpose. 

 As time elapsed a change came over the experiment. Little 

 by little, the rays began to be affected by the field. This 

 change was hastened by heating the active film in place 

 in the high vacuum. At the end the result was as un- 

 equivocally negative, all the rays being deviated by the 

 field in the highest vacua, as at the start it had been 

 positive. 



The clue, I th'nk, is the change of the glass surface of 

 the capillary, which it experiences under the excessive bom- 

 bardment to which it is exposed, and which is indicated 

 by the blackening of the glass. In the lead glass used it 

 was remarked independently that the darkening appeared to 

 commence somewhat suddenly. At the conclusion of the 

 experiment it was always marked. But on cutting down 

 the capillary b'fore the commencement in the three final 

 experiments with relatively short exposure to the emanation 

 the darkening had not commenced, whereas in the last 

 experiment, when the pole pieces were removed to allow 

 the deposit to be heated in place it was noted that the 

 darkening had begun. It can be imagined that the slightest 

 roughening of the surface is all that is necessary to cause a 

 negative result. The whole series of experiments from 

 start to finish is explained if accompanying the darkening 

 of the glass there is also a slight roughening. Whether 

 this will prove suflicient to be within the range of the 

 microscope remains to be seen. 



I hope to examine the hypothesis that the blackening 

 of the glass is accompanied by the roughening of the sur- 

 face more in detail later. But whether this or some other 

 explanation proves correct there can, I think, be no doubt 

 about the conclusion that the a particle has been isolated 

 under conditions in which it is not deviated by a magnetic 

 field, and, therefore, is not charged. The theoretical conse- 

 quences of the discovery need not here be dealt with. Cer- 



