CONTEMPORARY ADVANCES IN PHYSICS 83 



We turn to the rays themselves. 



The alpha-rays are particles of mass 6.60- lO"^^ gramme and positive 

 charge 2e or 9.55 • lO""^" electrostatic unit. The particles emitted from 

 dififerent radioactive substances differ, so far as we know, only in their 

 initial speeds. The range of variation is astonishingly small; the 

 slowest known alpha-particles issue from their sources (atoms of 

 uranium I) with a speed of 1.423-10^ cm/sec, the fastest ^^ emerge from 

 atoms of thorium C with a speed of 2 .069 • 1 0^ cm/sec. The differences 

 in speed between different alpha-particles emerging from a substance 

 are imperceptibly small. 



As a rule the speed of the alpha-rays from a substance is neither 

 measured nor quoted directly; one measures by preference their range 

 in air, a thing which can be defined because alpha-particles ionize air 

 (and other substances) more readily the more slowly they are moving, 

 until their speeds drop below 10^ cm/sec and they suddenly cease to 

 ionize altogether. Consequently, if alpha-rays shoot out from a bit of 

 radioactive substance into environing matter, the concentration of the 

 ions which they produce increases steadily and rapidly from the 

 emitting substance outwards, up to a distance where it attains a sharp 

 maximum and then suddenly falls to zero.^^ This distance is the range 

 in the material in question; it is greater the faster the alpha-rays, 

 varying as the cube of their initial speed. It is a property of the 

 alpha-rays and not of the substance which emits them, and I should not 

 have introduced it here but for a certain relation between ranges and 

 half-periods, and as a pretext for showing some pictures of pleochroic 

 haloes. 



These haloes occur in certain ancient minerals, chiefly mica; they 

 are systems of concentric spheres of discoloration, of which the 

 pictures represent cross-sections. No one could imagine what they 

 were when they were first discovered ; but the explanation is simple and 

 beautiful. Particles of uranium in some cases, of thorium in others, 

 bubbles of radon in yet others, were caught ages ago and held in the 

 points which were to become the centres of the haloes; the spheres of 

 discoloration are the regions of maximum intensity of ionization, where 

 the alpha-rays emitted from the central source were slowed down to 

 their speed of optimum ionizing-power and were on the verge of 



22 Among the particles issuing from samples of thorium C and producing scintilla- 

 tions on fluorescent screens, very occasional ones (one in ten thousand, or fewer) have 

 a much greater range than the rest, or than the characteristic particles of other sub- 

 stances. A few corpuscles of abnormally long range issue from samples of radium C. 

 It is a controversial question whether these particles come from nuclei disintegrating 

 in a rare and abnormal manner, or from nuclei struck and broken by alpha-particles 

 ejected from other atoms, as sometimes happens. Even the published data are not 

 all in accord, and it is unsafe to make further statements. 



23 Introduction, pp. 200-204. 



