DISCOVERY 



201 



helium and hydrogen atoms, and of certain electrons. 

 The number of helium nuclei may be small or great 

 according as the atom is light or heavy, but the number 

 of hydrogen nuclei is supposed to vary only from o to 3. 

 But we need not discuss these speculations further. 

 The essential point is that the nucleus of a radio- 

 element may expel part of itself in two ways : (i) it 

 I may expel the nucleus of a helium atom — thereby 

 I emitting energy (for the helium nucleus weighs 4 and 

 i travels at about 10,000 miles per second) and thereby 

 also becoming transformed or, if you like, transmuted, 

 into the nucleus of an entirely different element ; or 

 (2) it may shoot out an electron, which has, however, 

 much less energy than a helium nucleus (because it 

 has "almost no weight "), and after so doing become 

 an entirely different nucleus. Now, it is found that 

 whenever a helium-nucleus is emitted by a radio- 

 element, the new element formed lies two places lower 

 than the old in the ordinary way in which elements are 

 arranged, and whenever an electron is shot out the 

 new radio-element lies one place higher. This rule is 

 found to hold perfectly in radio-active transformations, 

 from the heaviest radio-elements doun to the inactive 

 element lead. There is no reason to think it would fail 

 to hold, therefore, for any body which might be com- 

 pelled to expel a helium nucleus or an electron. Let 

 us now arrange the elements from platinum to lead in 

 the order of their atomic weights. This order run? 

 platinum, gold, mercury, thallium, and lead. If lead 

 could be induced to expel a helium nucleus, the resultant 

 atom would be mercury (two places lower) ; by a 

 similar transformation platinum would arise from 

 mercury. If, finally, an atom of platinum could be 

 induced to expel from its nucleus an electron, gold 

 (one place higher) would result. 



But radio-activit}' teaches that the essential point 

 about this kind of transmutation is the energy pro- 

 duced and not the body produced as a result. It is 

 the energy alone that is obvious and important. The 

 gold or other bodies produced would hardly have the 

 importance even of by-products. A man looking for 

 gold as the result of the transmutation of lead is 

 more foolish than one who cannot see that the 

 importance of an internal-combustion engine lies in 

 making a motor-car go, because he is intent on study- 

 ing the waste gases shot out by the exhaust. A gram 

 of radium with its products gives out 130 units of heat- 

 energy every hour continuously; yet radium is a very 

 rare element, and its rate of transmutation is actually 

 very slow. Imagine the energy of radium if we had 

 pounds of it, and if its rate of transmutation could be 

 speeded up at will. How much greater would be the 

 energy evolved by common elements like lead or 

 mercury, procurable by the ton, if they could be made 

 to do by artifice what radium does spontaneously ! 



But so far the hea\y elements cannot artificially be 

 made to disintegrate, but experiments with a few of 

 the lighter ones show that the process, although 

 extremely difficult, is to some degree possible, and 

 these will now be described. 



Artificial Transmutation 



These experiments have been done by Sir Ernest 

 Rutherford at Manchester and at Cambridge, and the 

 method in outline is this. The nuclei of different 

 atoms are bombarded by swift a-particles (which are 

 helium nuclei) from the radio-element radium C ; 

 a minute fraction of the nuclei of certain elements is 

 found to be ruptured by these particles and a small 

 portion of a nucleus is broken off ; these bro ken-off 

 pieces are examined by an ingenious device and found 

 in all cases to be nuclei of hydrogen atoms. 



The nucleus of an atom, it has been mentioned, is 

 so smaU with respect to the atom itself that if the latter 

 were magnified to the size of an ordinary market-town, 

 the former would measure little more than a foot every 

 way. It is obvious, therefore, that to smash up the 

 nucleus — which alone insures artificial disintegration 

 — not only is a projectile of some kind necessary, but 

 particularly accurate shooting must be done. Since 

 there is no means of taking aim or correcting fire, the 

 only means of hitting the target is to fire a very large 

 number of projectiles. A few then have a chance of 

 hitting. 



The a-particle from radium C is a helium nucleus 

 with a mass of four units, shot out with a velocity of 

 about 12,000 miles per second. This is, of course, a 

 very tiny mass, but mass for mass it has about four 

 hundred million times the energy of a rifle buUet. 

 When it strikes a screen of crystallised zinc sulphide 

 it emits a flash of light, a scmtiUation, which may be 

 observed through a low-power microscope by eyes 

 well rested in the dark. The light seen thereby is 

 known to be due to the collision of a single nucleus 

 with the molecules of the zinc sulphide screen. It is 

 found, however, that, if the screen be moved away from 

 the source of a-particles farther than 7 cm. in air, 

 no light due to these particles can be observed. That 

 is because 7 cm. of air offers such resistance to the 

 passage of these nuclei that they lack sufficient energy 

 to cause a flash of light. If next these nuclei are 

 compelled to force themselves through a substance 

 much denser than air, saj' a thin sheet of magnesium 

 or of paper or of mica, they wiU naturally be stopped 

 much sooner, but they will go a distance before they 

 are completely stopped, which is roughly inversely 

 proportional to the sheet's density. Rutherford did 

 his first successful experiments with nitrogen, but a 

 later experiment with aluminium is more convenient 



