July 24, 1919] 



NATURE 



415 



three steps. Some 2 ix;r cent, (45 lb. per ton of seed) 

 i> recovered in the saw-linting machine as ' linters," 

 and about 3 per cent. (67 lb. per ton) in the seed- 

 defibrating machine as "seed-lint"; while some 12 per 

 cent. (112 lb. per ton of seed) is obtained in the huU- 

 defibrating machine as "hull-fibre." All three pro- 

 ducts now command high prices. Calculated on a 

 pre-war basis, the three grades aggregate 45s. per 

 ton of seed ; the cost involved is i is. bd. per ton ; and 

 the net e.xtra return is about 33s. per ton. 



The British milling system, which crushes the 

 entire seed, prevents complete recovery of the residual 

 fibre. Even so, and assuming that 2 per cent, of 

 fibre is left on the seed, 2 per cent, could still be 

 recovered as "linters" and 6 per cent, as "seed-lint." 

 The additional value should be 32s-. per ton of seed, 

 provided the recovery be effected in the oil-milling 

 operation. But it will be preferable, whenever pos- 

 sible, to defibrate the seed in the country of origin. 

 Were Uganda seed defibrated at the ginning in 

 Uganda there would result : — (a) A profit on the 

 "linters" and "seed-lint" recovered; (b) a reduction 

 of the space occupied by the defibrated exported seed, 

 with a consequent saving of 25 per cent, or more in 

 ocean freight; (c) a diminution of the liability of 

 cotton-seed to heat during the voyage and a con- 

 -sequent reduction in insurance rates; and (d) a prob- 

 able increase in the price paid for defibrated as com- 

 pared with "fuzzy" seed. At pre-war rates these 

 iactors, taken conjointly, should mean an increase of 

 505. per ton in the prices paid for Uganda seed in 

 the British market. 



COLLISION OF a- PARTICLES WITH 

 LIGHT ATOMS.^ 



THE discovery of radio-activity has not only 

 thrown a flood of light on the processes of 

 ' insformation of radio-active atoms;, it has at the 

 .iiie time provided us with the most powerful natural 

 i:_;incies for probing the inner structure of the atoms 

 of all the elements. The swift o-particles and the high- 

 speed electrons or jS-rays ejected from radio-active 

 bodies are by far the most concentrated sources of 

 energy known to science. The enormous energy of 

 the flving o-particle or helium atom is illustrated by 

 the bright flash of light it produces when it impacts 

 on a crvstal of zinc sulphide, and by the dense dis- 

 tribution of ions along its trail through a gas. This 

 great store of energy is due to the rapidity of its 

 motion, which in the case of the a-particle from 

 radium C (range 7 cm. in air) amounts to ig,ooo km. 

 per second, or about 20,000 times the speed of a rifle- 

 bullet. It is easily calculated that the energy of 

 motion of an ounce' of helium moving with the speed 

 of the a-particle from radium C is equivalent to 

 10,000 tons of solid shot projected with a velocity of 

 I km. per second. 



In consequence of its great energy of motion the 

 charged particle is able to penetrate deeply into the 

 structure of all atoms before it is deflected or turned 

 back, and from a study of the deflection of the path 

 of the a-particle we are able to obtain important 

 evidence on the strength and distribution of the electric 

 fields near the centre or nucleus of th<' atom. 



Since it is believed that the atom of matter is, in 

 general, complex, consisting of positively and nega- 

 tively charged parts, it is to be anticipated that a 

 narrow pencil of a-particle-;. after passing through a 

 thin plate of matter, shou'd be scattered into a com- 

 parativelv broad beam. Geiger and Marsden «.howed not 



I Di'course delivered at tVe Royal in«litut=on nn June 6 by Sir E- 

 Rutherford, F.R.<J. 



only that much small scattering occurred, but also that 

 in passing through the atoms of a heavy element some 

 of the a-particles were actually turned back in their 

 path. Considering the great energy of motion of the 

 o-particle, this is an arresting fact, showing that the 

 o-particle must encounter very intense forces in pene- 

 trating the structure of the atom. In order to explain 

 such results, the idea of the nucleus atom was 

 developed in which the main mass of the atom is 

 concentrated in a positively charged nucleus of very 

 small dimensions compared with the space occupied 

 by the electrons which surround it. The scattering of 

 o-particles through large angles was shown to be the 

 result of a single collision where the o-particle passed 

 close to this charged nucleus. From a study of the 

 distribution of the particles scattered at different 

 angles, results of first importance emerged. It was 

 found that the results could be explained only if 

 the electric forces between the o-particle and charged 

 nucleus followed the law of inverse squares for dis- 

 tances apart of the order of lo-" cm. Darwin 

 pointed out that the variation of scattering with velo- 

 city was explicable only on the same law. This is 

 an' important step, for it affords an experimental proof 

 that, at anv rate to a first approximation, the ordinary 

 'law of force holds for electrified bodies at such ex- 

 ceedinglv minute distances. It was also found that a 

 resultant charge on the nucleus measured in funda- 

 mental units was about equal to the atomic number 

 of the element. In the case of gold this number is 

 believed from the work of Moseley to be 79. 



Knowing the mass of the impinging o-particle and 

 of the atom with which it collides, we can determme 

 from direct mechanical principles the distribution of 

 velocities after the collision, assuming that there is 

 no loss of energv due to radiation or other causes. 

 It is important to notice that in such a calculation 

 we need make no assumption as to the nature of the 

 atoms or of the forces involved in the approach and 

 separation of the atoms. For example, if an 

 a-particle collides with another helium atom, we 

 , should expect the a-particle to give its energy to the 

 I helium atom, which could thus travel on with the 

 speed of the o-particle. If an o-particle collides 

 directlv with a heavv atom, e.g. of gold of atomic 

 weight 197, the o-particle should retrace its path 

 with onlv slightlv diminished velocitv, while the gold 

 atom moves onward in the orii.nnal direction of the 

 o-particle, but with about one-fiftieth of its velocity. 

 Next consider the important case where the o-particle 

 of mass 4 makes a direct collision with a hydrogen 

 atom of mass i From the laws of impact the 

 hvdrogen atom is shot forward with a v;elocity 

 1.6 times that of the impinging c-partide. while the 

 o-particle moves forward in the same direction, but 

 with onlv 06 of its initial speed. Marsden showed 

 that swift' hvdrogen atoms set in motion bv impact 

 with a-particles can be detected like o-oarticles bv 

 the scintillations produced in a zinc sulphide crvstal. 

 Recentlv I have been able to measure the speed of 

 such H atoms and found it to be in good accord WMth 

 the calculated value, so that we mav conclude that 

 the ordinarv laws of impact mav be applied with con- 

 fidence in such cases. The relative velocities of the 

 o-oarticles and recoil atom after collision can thus tie 

 simplv illustiated bv impact of two perfectly elastic 

 balls ■ of masses proportional to the masses ot the 

 atoms. ,. , ., 



While the velocities of the recoil atoms can be easilv 

 calculated, the distance which thev travel before being 

 brought to rest dep<MKls on both the mass and the charge 

 carried bv the recoil atorn. Experimen* shows that 

 the ran£/e of H atoms, like the ran^e of o-particles, 

 varies nearlv as the cubr of their initial velocity. If 



NO. 2595, VOL, 103] 



