February 9, 191 1] 



NATURE 



493 



abstraction of energy. There is clearly an actual absorp- 

 tion effect, which is to be distinguished from the scatter- 

 ing effect. Indeed, the two effects are obviously of 

 different importance in the two cases. When a ^ ray 

 strikes a gold atom it must be much more liable to 

 deflection than when it strikes the lighter atom of 

 aluminium. On the other hand, I think it can be shown 

 clearly that in ploughing through aluminium atoms there 

 is a relatively quicker absorption of energy. We may 

 illustrate this by a rough model. Let us stand an electro- 

 magnet upright on the table, and let us suspend another 

 magnet so that it can swing over the fixed one and just 

 clear it. If we draw back the swinging magnet and let 

 it go towards the fixed one, the currents running so that 

 the two repel, then as the moving magnet tries to go by 

 there will be a deflection depending on the relative speed, 

 the closeness- of approach, and the strength of the poles. 

 This may represent the turning aside of an electron by a 

 centre of force inside an atom. Now let the magnet at 

 the table be supported by a spiral spring so as to be still 

 upright, but have some freedom of motion ; then, when 

 the experiment is repeated, the swinging magnet pushes 

 the other more or less to one side ; it is less deflected, but 

 it has to give up some of its energy. This is exactly 

 •what happens in the case of the /3 particle. The centre 

 of force in the gold atom behaves like the stiffer electro- 

 magnet on the table ; it deflects the electron more, but 

 robs it of less energy in doing so. It will not do to sup- 

 pose the gold atom to diff^er from the aluminium atom 

 simply in the number of centres of force, such as electrons, 

 which it contains if it is supposed that they all act in- 

 dependently. There is some other fundamental difference, 

 equivalent to a diff^erence in the stiffness with which the 

 electrons are set in their places. There are two things 

 to be expressed in the behaviour of the atom towards the 

 /8 particle, as has been pointed out several times. H. W^ 

 Schmidt has actually calculated them from experiments 

 which gave them indirectly and somewhat approximately. 

 •The method I have just outlined gives one of them directly, 

 viz. that which is called the scattering coefficient, and I 

 think the other can also be found directly by a method 

 .which will serve as an illustration of the behaviour of 

 7 rays. 



We must first, however, consider the part which 7 and 

 X-rays play generally in this theory. Workers are by no 

 means agreed as to the proper way in which to regard 

 them, but there is no need to enter at once on a discussion 

 as to their nature. It is well known that they have the 

 most extraordinary powers of penetration, and are un- 

 affected by electric or magnetic fields. Thev have one 

 property which alone, as I think, brings them' within our 

 experience ; that is to say, the power of exciting /3 ravs 

 from the atoms over which they pass. Were it not for 

 this they would still be unknown. When we examine this 

 production of 3 rays, we find that in the first place their 

 speed depends on the quality of the 7 rays which cause 

 them, and not on the nature of the atoms' in which thev 

 arise ; in the second, that the & rays to a large degree 

 continue the line of motion of the 7 ravs, as if the latter 

 pushed them out of the atoms ; and', lastlv, that the 

 number of the 3 rays depends on the intensi'tv of the 7 

 rays. It is these facts which suggest the simple theorv 

 I have already described. The 7 ray is some minute 

 thing which moves along in a straight line without change 

 of form or nature, which penetrates atoms with far greater 

 ease than the a or /3 particle, which is not electrified, and 

 •which sooner or later disappears inside an atom, handing 

 on a large share of its energy to a j3 particle which takes 

 Its place. The absorption of 7 rays is simply the measure 

 of their disappearance in giving rise to )3 ravs, one 7 ray 

 producing one j3 ray, and no more. 



We find the same sort of scattering in the case of 7 

 rays as in that of & rays. Of a stream of ravs directed 

 against a plate which it can penetrate easilv, w'e find that 

 a few are turned completely back, a ver'v much larger 

 number are only slightly turned out of their path, and 

 the rest go on. The scattered rays are verv similar to 

 the original rays ; there is no need to suppose that the 

 original ray disappears, to be replaced by a secondary, anv 

 more than there is to suppose that a and j3 rays disappear 

 and are replaced by others in similar cases. When, there- 

 fore, a 7 ray enters an atom, three possibilities await it. 

 NO. 2154, VOL. 85] 



The first is a negative one ; it may go through the atom 

 untouched, and this must happen in the majority of cases ; 

 the second chance is that of deflection, and the third that 

 of conversion into a & ray, using the word conversion in 

 a general sense, without going into details as to the nature 

 of the process. 



Now we may consider our 7-ray problem. Suppose a 

 stream of these rays passing over a block of any sub- 

 stance, such as aluminium, or zinc, or lead. When they 

 are really penetrating rays they are equally absorbed by 

 equal weights of these materials, which means that in 

 equal weights equal numbers of 3 rays spring into exist- 

 ence. If these jS rays were able to move through equal 

 weights of the metals, we should find in each metal the 

 same " density " of /3 rays ; and the important point is 

 that this is independent of whether the rays are straight 

 or crooked in their paths. If ten lines of given length 

 were begun in every square centimetre of a sheet of paper, 

 the ink used in drawing them would be independent of 

 the straightness of the lines, but proportional to their 

 length. Now if we make a cavity in each metal the 3 

 rays will cross it in their movements to and fro, and if a 

 little air is introduced into the cavity, the ionisation pro- 

 duced in it will be a measure of the density of the /3 rays, 

 and therefore the average distance each moves in the 

 metal. Experiment shows that we get twice as much 

 ionisation in a cavity in the lead as in a similar cavity 

 in the aluminium, and we conclude that the jB particle 

 really has a longer track in the heavier metal. This 

 experiment gives us the second constant of i3-ray absorp- 

 tion, that is to say, the rate at which its energy is taken 

 away from it ; the other experiment gave the chance of 

 deflection only. We see that the path of a 3 ray in 

 aluminium is more direct, but of less length, than in lead; 

 in the latter metal it has really a longer path, but it does 

 not get so far away from its starting point because it 

 suffers so many more deflections. 



Finally, let us take a problem from the X-rays. Let 

 us see how we may test the idea that X and 7 rays do not 

 ionise themselves, but leave all the work to be done by 

 the jS rays which they produce. Suppose a pencil of 

 X-rays to pass across a vessel and to produce ionisation 

 therein. It is convenient to use, not the original X-rays, 

 which are heterogeneous, but the rays which are scattered 

 by a plate of tin on which the primary rays fall. Such' 

 " tin ravs," as we often call them briefly, are fairly 

 homogeneous, and give kathode rays of convenient pene- 

 tration. In some experiments of mine the rays crossed 

 a layer of oxygen 3-45 cm. wide, having a density 000137, 

 and the ionisation oroduced was 227 on an arbitrary scale. 

 The result may be put in the following way. Suppose, 

 provisionally, that all this ionisation is done indirectly ; 

 the oxygen has converted so much X-ray energy into 

 kathode-rav energy, and these kathode rays penetrating 

 their one or two millimetres of oxygen, which is all they 

 can do, have ionised the gas. Then we may say that, in 

 crossing a layer of oxygen weighing 345 x 000137. or 

 0-00473 gr. per sq. cm., enough kathode rays have been 

 produced to cause an ionisation of 227 units, and there- 

 fore that a layer weighing one milligram per sq. cm. 

 would produce 48 units in the same way. We now pro- 

 ceed to compare this production in oxygen with the similar 

 effect in a metal such as silver. Stretching a silver foil 

 across the chamber in the path of the rays, we find that 

 under the same intensity of ravs the ionisation is largely 

 increased, and the change is due to kathode rays which 

 the X-rays have generated in the silver. Not all these 

 ravs get out of the silver, but we can overcome this 

 difficulty by taking silver foils of different thickness, draw- 

 ing a curve 'onnecting the effect of the foils with their 

 thicknesses, taking the cur\'e back to the origin, and so 

 finding what would be the effect of a foil so thin that all 

 the kathode rays did get out. In my case I found that 

 a milligram of silver produced enough kathode rays to 

 give an ionisation 1580. This is thirty-three times as 

 much as the oxygen could do. Now. according to our 

 theory, this should be because silver absorbs tin ra^-s 

 thirty-three times more than oxygen does, and exoeriment 

 showed this to be very nearly the case. In finding the 

 absorbing power of oxygen, I measured first those of 

 carbon and oxalic acid, and then proceeded by calculation, 

 for the absorption in a gas is difficult to determine. 



