May 6, 1922] 



NATURE 



i85 



times greater than that of a swift rifle bullet. Mass 

 for mass, its energy of motion is four hundred million 

 times greater than that of the bullet. 



Whilst no doubt an a-particle fired directly at a 

 heavy nucleus may penetrate its structure, its energy 

 may at that stage be too small to cause a disruption. 

 The attack on the lighter atoms is much more promis- 

 ing, for the repulsive forces are so much smaller that 

 the a-particle may still retain much of its energy on 

 entering the nuclear structure. 



Before, however, considering experiments on this 

 question, it is desirable to say a few words on the 

 collision of a-particles w^ith hydrogen nuclei, where no 

 question arises of the disruption of the atom. When 

 a-particles pass through hydrogen gas, there are 

 occasional close collisions between the a-particles and 

 the hydrogen nuclei, resulting in the appearance of 

 high speed H-nuclei. These H-particles travel about 

 four times the distance of the bombarding a-particle, 

 and can be detected easily by the scintillations they 

 produce on a zinc sulphide screen. From the ordinary 

 principles of mechanics, the maximum speed given to 

 an H-nucleus is i'6 times that of the colliding a- 

 particle, whilst the maximum energy communicated 

 to it is 0-64 of the energy of the a-particle. It is found 

 that the number of these swift H-atoms is far in excess 

 of that to be expected if it be supposed that the a- 

 particle and hydrogen nucleus behave as point charges 

 for the very small distances involved in these violent 

 collisions. In addition, the variation of the number 

 with the velocity of the a-particle and the number 

 shot ofif at different angles with the direction of the 

 tt-particle differ markedly from the results to be expected 

 on the simple point theory. 



It seems clear that not only has the a-particle a 

 structure, but that the law of force at very short 

 distances is entirely different from that of the inverse 

 square. As a result of a careful investigation, Chad- 

 wick and Bieler concluded recently that the results of 

 the collisions could be explained by supposing that the 

 ('.-particle — to which the complexity is ascribed — 

 behaves like a spheroid of axes 8 x lo"!^ and 5 x lo"^^ 

 cm. Outside this surface, the law of the inverse 

 square applies, but the forces increase so rapidly when 

 the H nucleus enters the spheroidal surface that it is 

 rapidly turned back. This model of the helium nucleus 

 is, no doubt, quite artificial, but it gives us som.e idea 

 of its probable dimensions and the extent of the region 

 in which new and powerful forces come into play. 



We should consequently anticipate that, in a close 

 collision of a swift a-particle with the nucleus of an 

 atom more complex than that of hydrogen, the ordinary 

 laws of force would break down when the distances 

 separating the particle and nucleus became very small. 

 It must be remembered that gigantic forces come into 

 play in these nuclear collisions, and only very stable 

 structures may be expected to survive the encounters. 

 The first observation which has to do with the main 

 subject of my lecture was made some years ago. When 

 the a-rays from a strong radioactive source pass through 

 dry gases like oxygen or carbon dioxide, a small number 

 of weak scintillations are observed on a screen beyond 

 the range of the a-particles. These " natural " scin- 

 tillations are believed to be due to atoms of hydrogen 

 coming from the source, and probably result from a 



NO. 2740, VOL. 109] 



slight hydrogen contamination of the source during 

 exposure to the radium emanation. If, however, dry 

 air is substituted for oxygen or carbon dioxide, the 

 number of scintillations is increased three or four times. 

 This additional effect was found to be due to the pre- 

 sence of nitrogen, and was shown in a correspondingly 

 greater degree by chemically prepared nitrogen. By 

 suitable arrangements, it was found that the particles 

 causing these scintillations were bent by a magnetic 

 field to about the extent to be expected if they con- 

 sisted of swift, charged H-atoms. It seemed probable 

 from the beginning that these additional H-atoms, 

 which appeared only in dry nitrogen and not in oxygen 

 or carbon dioxide, must have their origin in a dis- 

 integration of the nitrogen nucleus by collision with a 

 swift a-particle. 



With the original counting arrangements, the scin- 

 tillations were small in number, weak in intensity, and 

 difficult to count with accuracy. Further progress has 

 depended mainly on improvements in the counting 

 microscope, with the object of increasing the intensity 

 of the scintillations and the area of zinc sulphide screen 

 under observation. By the use of wide-aperture ob- 

 jectives and special eyepiece lenses of low magnifying 

 po\^er, the counting of these scintillations has become 

 much easier and more definite. 



We shall now consider the methods adopted to in- 

 vestigate in more detail the effects observed in nitrogen 

 and to test whether other elements behave in a similar 

 way. The apparatus required is of the simplest 

 character and consists merely of a brass tube, 3 cm. 

 in diameter, provided with stopcocks by means of 

 which dry gases may be circulated through it. At 

 one end of the tube is a hole covered with a thin silver 

 plate. The zinc sulphide screen is fixed 1-3 mm. 

 away from the opening, leaving a slit in which absorb- 

 ing screens of mica can be inserted. The radioactive 

 source is fitted on the end of a rod so that its distances 

 from the screen can be varied at will. In order to 

 reduce the luminosity due to the /?-rays from the 

 source, the whole apparatus is placed in a strong 

 magnetic field. It may be of interest to give a few 

 details in illustration of the magnitude of the effects 

 to be expected under different conditions. Suppose 

 that the radioactive source, consisting of a brass disk 

 coated on one side with an invisible layer of radium-C 

 corresponding in y-ray activity to 40 milligrams of 

 radium, is placed 3-5 cm. from the screen and that a 

 current of dry hydrogen is passed through the ap- 

 paratus. Suppose the stopping power of the materials 

 between the source and the zinc sulphide screen 

 corresponds to 20 cm. of air, that is, it would suffice 

 to stop an a-particle of range 20 cm. in air. The 

 passage of the a-particles, which in this case have a 

 range of 7 cm., through the hydrogen liberates a large 

 number of high-speed H-atoms, which produce scintil- 

 lations on the screen. Their number, seen through 

 a special microscope which has a field of view of 40 

 sq. mm., is so great — thousands a minute — that it 

 would be impossible to count them without reducing 

 the activity of the source. As additional absorbing 

 screens of mica are added, the numbers fall off rapidly, 

 and for an absorption of, say, 30 cm. not a single 

 H-scintillation can be observed per minute. A similar 

 effect is shown if oxygen is substituted for hydrogen 



