CONTEMPORARY ADVANCES IN PHYSICS 631 



transmutation beyond the shadow of a doubt. There was of course no 

 certainty beforehand that he would succeed. On the contrary there 

 were apparently grave grounds for pessimism. We must take note of 

 these; the proof that they were not justified is not the least important 

 part of Rutherford's achievement. 



First, even the energy of the alpha-particles might have been too 

 small to injure a nucleus. Indeed, for many kinds of atom-kernels it 

 is too small, to judge from the work of Rutherford's school at the Caven- 

 dish Laboratory; and for the rest there is not much margin to spare; 

 from the work of that school it appears that if the fastest alpha-particles 

 moved with a speed as great as six- tenths as their actual speed, and 

 no greater, the effect would never have been discovered. 



Second, there was reason to fear that the nuclei are too small to be 

 struck except by the rarest of chances, too rare a chance to be service- 

 able. The observations on deflected alpha-rays had proved that the 

 kernels of atoms are less than IQ-^^ cm. across, the impinging particles 

 no greater: a very thin missile and a very tiny target! Had they been 

 a few orders of magnitude smaller than this maximum limit, "square 

 hits" would have been too few to notice. As a matter of fact, in the 

 first of the successful experiments, the proportion of these was about 

 one to every million of alpha-particles traversing the layer of nitrogen 

 gas which Rutherford was trying to transmute. 



Third, the fragments of the broken nuclei might not have been 

 observable. Delicate as are the methods of chemical analysis, they 

 are not fine enough to detect alterations so Infrequent as these were 

 expected to be, and were actually found to be. Alpha-particles them- 

 selves are detected in three ways— by the luminous splashes or "scin- 

 tillations" which they cause when they Impinge on fluorescent screens; 

 by the trails of water-droplets which they leave behind them when 

 they dash through moisture-saturated air which is suddenly cooled 

 just before or just after their passage; and by the electrical discharges 

 (small-scale sparks) which they touch off when they pass through air 

 in the neighborhood of a charged and sharply-pointed needle. The 

 two last of these are due to ions which the particle forms by detaching 

 electrons from molecules of the gas. The slower the particles, the 

 fewer the Ions; the less conspicuous are these effects and the more likely 

 to be missed. As for the scintillations we know but little of their 

 mechanism, but we do know that the slower the particles, the fainter 

 the flashes. Thus it is altogether reasonable to suppose that when 

 nuclei are broken into fragments, the fragments may be moving too 

 slowly to be noticed by any of these three procedures! (One might 

 even suspect that the pieces of a fractured nucleus may not have 



