492 



NATURE 



[February 9, 191 1 



porary. For only a very short time can any ray be 

 described as matter in a fourth state ; at the end of it the 

 extraordinary condition has terminated, the particle has 

 lost its tremendous speed or suffered some other change, 

 and the ray ceases to exist. Speaking technically, we are 

 dealing with initial, not permanent, conditions. 



Let us now come back to resemblances between the two 

 kinds of motion, for there is one point of similarity which 

 is not quite so obvious as others I have mentioned, and 

 is, I think, of the greatest importance ; in fact, it is 

 largely on account of this similarity that I have ventured 

 to put the two theories together for comparison. 



When the first experimenters in radio-activity allowed 

 their streams of rays to fall upon materials of various 

 kinds, they found that the irradiated surfaces were the 

 sources of fresh streams of radiation. The secondary rays 

 were sometimes of the same nature and quality as the 

 primary, sometimes not. Further, they found that the 

 secondaries, on striking material substances, could pro- 

 duce tertiaries, and so on. The examination of all the 

 variations of this problem — the investigation of the con- 

 sequences of changing the primary-, of changing the sub- 

 stance, and last, but not least, of changing the form of 

 the experimental arrangements — has been the cause of an 

 enormous amount of work. There is a large literature 

 dealing with secondary radiations of all kinds which, I 

 imagine, but few have read with any completeness, and 

 the subject has become, on the surface at least, compli- 

 cated and difficult. Now I believe that it is possible to 

 clear away the greater portion of this complexity at a 

 stroke by the adoption of an idea which makes it possible 

 to describe and discuss the whole of these phenomena in 

 a very simple way. When an encounter takes place 

 between two gas molecules, we suppose that the sum of 

 the energies of the two is the same after the collision as 

 before, and, further, that there are just two things to 

 consider — two molecules — after as well as before. I think 

 that we may carry this idea over almost bodily to the 

 newer theory. A radiant particle encounters an atom. 

 The particle is a definite thing ; it contains a definite 

 amount of energy, and whether it is an a, or jS, or 7, or 

 X-ray, its energy is to be found almost entirely inside a 

 very minute volume. The encounter takes place. When 

 it is over there are still two things, an atom and a radiant 

 particle, going away from it. The sum of the energies 

 of the two is still the same, which means that we denv 

 a possibility much considered at one time, viz. that in the 

 encounter the atom could be made radio-active, and could 

 unlock a store of energy usually unavailable. We suppose 

 there is no energy to be considered except the original 

 energy of the radiant particle, and we suppose that there 

 are not now two or more radiant particles in place of the 

 original one, which also is a limitation on previous ideas. 

 It is a theory which ascribes a corpuscular form to all 

 the radiations. Each particle, o, ;8, y, or X, is to be 

 followed from its origin to its disappearance, and we have 

 nothing to think of but the one particle threading its wav 

 through the atoms. It loses energy as it goes, though 

 little at any one collision, and it passes out of our reckon- 

 ing_ when it has lost it all. There are no secondary 

 radiations other than radiant particles moving in direc- 

 tions which are different from those in which thev moved 

 at first. Even ' when a kathode rav excites an X-rav in 

 the ordinary Rontgen tube, or the X-ray excites a kathode 

 ray in a manner almost as well known, it is hardlv an 

 exception to this rule. The kathode ray has an encounter 

 with an atom and disappears ; simultaneously the X-rav 

 comes out of the atom, a circumscribed corpuscle carrv'- 

 ing on the energy of the kathode ray. There is a change, 

 but it extends only to the external characteristics of the 

 carrier of energy. The X-ray passes through the glass 

 wall of the X-ray bulb, or at least it does so sometimes ; 

 it may pass through other matter as well, but sooner or 

 later it has a fatal encounter with an atom, and the 

 reverse change takes place. In all cases, in that of the 

 undeviatrng a ray, or the j3 ray which suffers so manv 

 deflections, or the 7 or X-rays, it is a matter of tracing 

 the movements of individual minute quantities of energy 

 until they finallv melt away. 



Let us consider one or two simple experimental results 

 from this point of view in order that we mav illustrnt*^ 

 this corpuscular theorv. and at the same time mav learn 

 NO. 2154, VOL. 85] 



something of the properties of the corpuscles and of the 

 arrangements of the atoms through which they pass. 



We take first one of the simpler cases, the movement 

 of an o particle through a gas. The relatively large mass 

 of the particle gives it an effectiveness which the other 

 radiations do not possess. It moves straight through 

 every atom it meets, and ionises most of them. Very 

 rarely does it suffer any deflection from its course until 

 its velocity is nearly run down. Then, indeed, it does 

 appear to depart considerably from the straight path, and 

 it may be that it is much knocked about by collision^ 

 before it finally comes to comparative rest. In this wa\ 

 we may explain the distribution of the ionisation along 

 its path, which increases slowly at first and rapidly after- 

 wards, until the a particle has nearly finished its journey ; 

 it then falls off rapidly. Considering that the ionisation 

 increases as the particle slows down and . spends more 

 time in each atom, and considering the more broken nature 

 of the path near its end, the reason of these peculiarities 

 !s clear enough. Apart from its comparative simplicity, 

 there are some other very interesting features of the 

 particle's motion. It is found, for example, that the loss 

 of energy which the particle incurs in crossing an atom 

 is proportional to the square root of the atomic weight 

 very nearly, and there is no certain explanation as yet of 

 this curious law. And again, Geiger has examined th' 

 small scattering that does occur, and found that o particle- 

 when moving quickly may be swung round completeh 

 even by the thinnest films of gold leaf, though the numher 

 is so small that the effect would have remained undetected 

 had it not been for the scintillation method which he and 

 Rutherford have perfected. He has found that about one 

 particle in 8000 is returned in this way from a gold plate, 

 which need consist only of a few thicknesses of gold leaf 

 in order to give the maximum effect. 



Now let us take an example from the behaviour of 

 the /8 rays. The /3 particle is so light that it is easily 

 deflected, even though it moves several times as fast as 

 the heavier o particle. Because it therefore possesses 

 little energy its effects are much smaller, and no one has 

 yet succeeded in handling a single j3 particle in the same 

 way as Rutherford and Geiger have handled the other. 

 We are obliged to content ourselves with observations of 

 the effects of a crowd of j3 particles, since the combined 

 action of many is necessary to give us an observable 

 result ; and at the same time that the /3 particle gives 

 much less effect than the a, it has a much more irregular 

 course, so that the problem is doubly difficult. We are,, 

 in fact, only just beginning to understand it. There is a 

 compensation in the fact that its very liability to deflec- 

 tion makes it all the more interesting an object. It is 

 possible — and this is the particular j8-ray problem I wish- 

 to consider now — to examine the deflection of a single 

 )8 particle by a single atom ; the parallel result in the 

 kinetic theory of gases has never, of course, been achieved. 



Suppose that we project a stream of j3 rays against a 

 thin plate and measure the relative number sent back, 

 which we do by measuring the ionisations caused by the 

 incident and returned rays respectively. We do this for 

 varying thicknesses of the plate, and plot the results, as, 

 for example, Madsen has done. His plate was made of 

 gold leaves, which could be had of extreme fineness. 

 From the relation thus obtained, it is possible to obtain 

 with confidence the amount of 3 radiation that would be 

 returned by the thinnest plate that could be imagined, 

 only one molecule thick. In such case the particles turned" 

 back could have had but one collision, and we have 

 achieved our purpose. Madsen 's figures show that a plate 

 weighing 4 milligrams to the square centimetre turned 

 back a tenth of the ;8 particles that fell upon it, and, so 

 far as can be judged, the ratio of the proportion turne«f 

 back to the weight of • the plate would be almost doubled 

 for very thin plates. We could go more into detail, an<f 

 find the distribution of those that are returned ; we should 

 then have data from which we might determine in some 

 measure the distribution of the centres of force inside the 

 atom. W^e cannot follow this up now, but I would like 

 to direct \-our attention to a curious indication which we 

 obtain when we compare the results for gold with those 

 which Madsen found for aluminium. They show that the 

 lighter metal turns back fewer j8 particles, and that its 

 power of absorbing a stream of rays is rather an absolute 



