34 SECTIONAL ADDRESSES 



will, and with very remarkable results. For example, the bombardment 

 of lithium with high velocity protons results in the formation of a-particles, 

 a process which may be described by saying that the lithium nucleus 

 whose atomic mass-number is 7 when bombarded by a proton whose 

 mass-number is 1, gives rise to two a-particles, each of mass- number 4. 



With this advance in technique has come a corresponding advance in 

 discovery. Thus the bombardment of a light element such as beryllium 

 by a-particles results in the production of y-rays together with a radiation 

 which does not ionise the air through which it passes, but may be recog- 

 nised by its effect on the nuclei which it itself bombards, producing, as 

 it does, ionisation tracks due to the protons expelled from these nuclei. 

 We have to deal, then, with a massive uncharged particle, whose mass 

 may be deduced from a study of the tracks made by the nuclei with 

 which it collides. The mass of the particle is very nearly equal to that 

 of the proton, and it has been called the neutron. 



For long it has been known that radiation of high penetrating power 

 exists in the atmosphere, a radiation which increases in intensity, that 

 is, in its power to discharge an electroscope, with increasing height. 

 This is the so-called cosmic radiation, which may be assumed to have its 

 origin in interstellar space. Investigations on cosmic radiation, using the 

 Wilson cloud chamber placed in a strong magnetic field, disclosed the 

 fact that when cosmic radiation passed into such a chamber tracks were 

 produced, some curved in one direction, some in the opposite sense. 

 This opposite curvature might be produced by a reversal of the sign of the 

 charge or it might be due to the fact that the particle was moving in a 

 direction opposite to that of its fellows of opposite curvature. It was not 

 difficult to rule out this latter possibility, and we are thus provided with 

 another sub-atomic entity of mass equal to that of the electron, and with 

 a positive charge equal to the electronic charge. 



This is the positron. 



The identification of heat and energy — a commonplace to-day — was, 

 as we have remarked, not established without difficulty. The twentieth 

 century has seen a possibly more remarkable identification — that of mass 

 and energy — an identification which was made, to within a factor of |-, 

 by Hasenohrl and was put forward in its present form in 1905 by 

 Einstein. In this form the energy (E) possessed by a mass (w) is given 

 by E = mc 2 , where c is the velocity of light. Increase of mass of a system 

 means increase of energy and conversely. And if mass be destroyed a 

 corresponding amount of energy appears as radiation, if conservation 

 laws hold. These conservation laws have been arrived at from a study 

 of large-scale phenomena, and there is no a priori reason why they should 

 be expected to hold when applied to atomic happenings far outside the 

 perceptual scheme of things. Indeed, one is tempted to ask, Why should 

 the concept of energy have any meaning, let alone any validity, when 

 applied to such systems ? The necessary and sufficient answer is the 

 pragmatic one. 



The possible invalidity of this law of conservation is no new concept. 

 Twelve years ago Bohr and his colleagues put forward a theory in which 

 an atom in an excited state emits radiation continuously, radiation which, 



