November 5, 1908] 



NA TURE 



13 



tion, and are the a particles atoms of a known element 

 or some new kind of matter? 



These problems were attacked by determining the 

 velocity and the value of E/M — the ratio of the charge 

 carried by an a particle to its mass — of a- particles 

 expelled from different kinds of matter. These quan- 

 tities can be determined by measuring the deflection 

 of a pencil of a rays when passing through strong j 

 magnetic and electric fields. E.xperiments of this 

 kind, which are difticult on account of the small 

 deflection of the a rays under normal experimental 

 conditions, have been made by Rutherford, Des 

 Coudres, Mackenzie, and Huff. The former deter- 

 mined the velocity and value of E/M for each of 

 a number of products of radium and actinium, while 

 Rutherford and Hahn made similar measurements for 

 some of the products of thorium. The results were 

 of great interest, for while it was found that the 

 initial velocity of projection of the a particles from 

 different kinds of matter varied from about 14,000 

 10 10,000 miles per second, the value of E/M was 

 the same for all. This shows that the a particle, 

 whether expelled from radium, thorium, or actinium, 

 is identical in mass and constitution, and that all the 

 radio-active substances which emit a particles have 

 a common product of disintegration. As the result of 

 a number of experiments, Rutherford found that the 

 value E/M for the a particle was 5070 in electro- 

 magnetic units. Now, from experiments on the 

 electrolysis of water, it is known that the correspond- 

 ing value of e/jn for the hydrogen atom is q6oo, 

 or nearly twice as large. The charge e carried by the 

 H atom is believed to be the fundamental unit charge 

 of electricity, so that the charge carried by any 

 bodv must be an integral multiple of e. If we suppose 

 the charge carried by an a particle is equal to the 

 charge carried by an hydrogen atom, the mass of the 

 a particle is, in round numbers, twice that of the 

 hydrogen atom, i.e. is equal to the molecule of 

 hydrogen. If, however, we suppose that E = 2C, i.e. 

 the a particle carries two unit charges, the mass of 

 the a particle is equal to about four. Now, it is 

 known that the atomic mass of heliuin is 3'g6 in 

 terms of hydrogen, so that on this supposition the 

 a. particle would appear to be an atom of helium 

 carrying two unit charges. We must now consider 

 some indirect evidence bearing on the question. As 

 the result of the experiments of Ramsay and Soddy 

 and others, it is now well substantiated that helium 

 is produced from radium. Debierne has shown that 

 helium is produced also from actinium. Unless the 

 helium is the result of the accumulated a particles, 

 it is diflicult to account for the production of the 

 helium observed. In addition, as we have shown, 

 the n particle is the only known common product 

 of the disintegration of radium and actinium, which 

 both give rise to helium. For these and other reasons, 

 Rutherford suggested in 1905 that it was very 

 probable that the a. particle was an atom of helium 

 carrying two unit charges. It has been found ex- 

 ceedingly difficult experimentally either to prove or 

 •disprove the correctness of this hypothesis, although 

 the settlement of this question has been for the last 

 few years the most important problem in radio- 

 activity, for, as will be seen, the proof that the o par- 

 ticle is an atom of helium carries numerous con- 

 sequences of the first importance in its train. 



We shall now describe some novel experiments by 

 Rutherford and H. Geiger, which have not only 

 thrown further light on this question, but have led 

 to important conclusions in several directions. An 

 account of this work is contained in two papers 

 published in the Proceedings of the Royal Society, 

 entitled " An Electrical Method of Counting the a 



NO. 2036, VOL. 79] 



Particles from Radio-active Matter," and "The 

 Charge and Nature of the a Particle " (A. vol. Ixxxi., 

 141-174, 1908). 



In the first paper an account is given of a method 

 for the detection of a single a particle and for counting 

 the number of a particles emitted from one gram of 

 radium. 



The current due to the ionisation of the gas pro- 

 duced by a single a particle is too small to detect 

 except by exceedingly refined methods. To overcome 

 this difficulty, recourse was had to a method of auto- 

 matic magnification of this current, based on the 

 ptinciple of generation of ions by collision — a subject 

 which has been investigated in detail by Townsend 

 and others.. Space does not allow us to enter into a 

 description ^of the methods employed for this purpose 

 or of the various experimental difliculties that arose 

 during the investigation. The general method em- 

 ployed was to allow the a particles to be fired through 

 a small opening into a detecting vessel containing 

 gas at low pressure exposed to an electric field not far 

 from the sparking value. The entrance of an a par- 

 ticle into the detecting vessel was marked by a sudden 

 ballistic throw of the electrometer needle. By adjust- 

 ment of the electric field, it was found possible to 

 obtain so large a magnification that the entrance of a 

 single a- particle was marked by a large excursion of 

 the electrometer needle. 



•n this way the expulsion of a particles was 

 detected from uranium, thorium, radium, and 

 actinium. In order to count accurately the number 

 of a particles expelled from one gram of radium, not 

 radium itself, but its product radium C vifas used as 

 a source of radiation. K surface was coated with a 

 thin film of radium C by its exposure for some hours 

 in the presence of the radium emanation. The use of 

 radium C as a source of rays had several advantages, 

 especially as regards the ease and certainty of measure- 

 ment of the amount of active matter present by means 

 of the 7 rays. The number of a particles passing 

 through an opening of known area at a known dis- 

 tance from the active source was counted for a definite 

 interval by noting the excursions of the electrometer 

 needle. From this the total number of a particles 

 expelled per second from the source was deduced. In 

 this way it was found that 3'4 x io'° a- particles were 

 expelled per second from the radium C present in 

 one gram of radium in equilibrium. It is known 

 from other data that radium itself and each of its 

 products, viz. the emanation, radium A and radium C, 

 expel the same number of a particles per second when 

 in equilibrium. Consequently in one gram of radium 

 in equilibrium 3'4 x 10'° a particles are expelled from 

 each of the products per second, and the total number 

 expelled is i'36 x 10" per second. On the most 

 probable assumption, that one atom of radium in 

 breaking up emits one a particle, 3'4 x 10'° atoms of 

 radium break up per second per gram. 



It was a matter of interest to compare the number 

 of scintillations observed on a properly prepared screen 

 of zinc sulphide with the number of o particles 

 striking it. Within the limit of experimental error, it 

 was found that the number of scintillations was equal 

 to the number of impinging a particles counted by 

 the electric method. Consequently each tt particle on 

 striking the screen produces a scintillation. It is thus 

 obvious that, using proper screens, the scintillation 

 method as well as the electric method may be em- 

 ployed to count the number of a particles emitted by 

 a radio-active substance. 



Apart from the importance of these results for 

 radio-active data, the experiments are of themselves 

 noteworthy, for it is the first time that it has been 

 found possible to detect a single atom of matter. 



