CONTEMPORARY ADVANCES IN PHYSICS, XXVIII 585 



sufficient for a-particles of energy up to and even beyond 10.6 MEV 

 (millions of electron-volts) and range up to and even beyond 11.5 cm. 

 Figure 6 exhibits the outward aspect, Fig. 7 the cross-section of this 

 device (one sees how the armature is fully contained within the rings). 

 The annular space and everything within it was evacuated (being 

 walled in by the ring B seen in the figures); the detector — a simple 

 ionization-chamber connected through a linear amplifier to an oscillo- 

 graph — ^was set 180° around the annulus from the source. This 

 device in the hands of Rutherford, Lewis and Bowden proved itself 

 able to furnish even a better spectrum than the scheme of the differ- 

 ential ionization-chamber; all of the peaks indicated by the former 

 curve were clearly separated, a hump which had suggested two groups 

 was resolved into three maxima, and an extra group was discovered — 

 twelve altogether! (Incidentally, the empirical energy-w.-range curve 

 of a-particles had previously been extended with the same device by 

 Wynn-Williams and the rest, to energy = 10.6 MEV and range 

 = 11.6 cm.) 



The long-range spectrum of RaC is thus of no mean complexity. 

 There will be occasion later for quoting its actual energy-values. 

 Of the long-range spectrum of ThC there is relatively little to be said; 

 evidently it has not been studied so intensively as the other, but it 

 seems to be comparatively simple, for only two groups have been 

 recognized. One of the groups of ThC has about the same range as 

 the highest group of RaC, so that between them they comprise the 

 most energetic subatomic particles ever yet discovered (about 10.6 

 MEV) apart from those of the cosmic rays and those resulting from 

 certain artificial transmutations. As for AcC, it is one of the con- 

 stituents of the mixture of radioactive bodies known as actinium 

 active deposit, from which a-particles of a range of about 10 cm. 

 have lately been observed in the Institut du Radium. 



Thus far I have written as though RaC and ThC were isolable 

 substances, of which one may obtain pure samples and analyze at 

 leisure the a-rays thereof. The truth, however, is far otherwise; for 

 the difficulties of making one radioactive substance practically free 

 from others, serious in most cases, are utterly insuperable in these. 

 Both RaC and ThC are so very ephemeral (their half-lives are too 

 small to measure, and are guessed from the Geiger-Nuttall relation as 

 10~^ and 10~^^ second respectively) that they can never be dissevered 

 from their mother-elements RaC and ThC which are also a-emitters. 

 Sometimes one finds the long-range particles designated as belonging to 

 RaC or ThC, and indeed I have nowhere found stated any compelling 

 reason for attributing them to the C-elements rather than the C- 



