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identical spectra. For this element, lead, Rutherford and Andrade have shown that the secondary y-radiation excited by the impact of B-rays on a block of ordinary lead gave by crystal reflection two lines identical in wave-length with the two strongest lines in the y-ray spectrum of radium-B, an isotope of lead, as Fleck showed, of atomic weight 214. This is of importance as indicating that X-rays and y-rays, although no doubt originating in a deeper region of the atom than the ordinary light spectrum, do not originate in the deepest region of all to which the weight of an atom and its radioactive properties are to be referred.

DESCRIPTION OF THE FIGURE.

The generalisation, brought up to date, is set forth in detail in the Tables on p. 134 and is illustrated by the accompanying figure, which is to be read at an angle of 45°, making the lines of atomic weight horizontal and the division between the successive places in the periodic table vertical. Starting from uranium and thorium, the series run in an alternating course across the table and extend over the last twelve places as far as the element thallium. At this point, it is interesting to note that the expulsion of an a- instead of a B-particle would have resulted in the production of an isotope of gold, and so literally have realised the goal of the alchemist. As it happens, a B-particle is expelled and lead results, so far as the changes have yet been traced, in all cases as the final product.

It has been necessary, in order to separate the series from one another, to displace the actinium series to the right and the radium series to the left of the centre of the places, but this displacement within the single place is not intended to express

DESCRIPTION OF THE FIGURE

133

any physical significance; but for the fact that many members would be superimposed, they would all be represented in the centre of the places.

The periods of average life, which are always 1.443 times the periods of half-change, are shown for each member above or below its symbol, a ? indicating that the period is estimated indirectly from the Geiger-Nuttall relation.

The figures at the head of each place represent the atomic numbers or number of the place in the periodic table, starting with hydrogen as unity, helium as 2, lithium as 3, and so on. Moseley found that the square-root of the frequency of the characteristic X-radiation of an element was, for the K-series of radiations, proportional to integers less by one than the atomic numbers. Strictly speaking, there is no means of determining the absolute value of the atomic number, but the starting point having been fixed for any one element, the others can then be found in terms of it. Moseley assumed the atomic number of aluminium as 13, as it is the thirteenth known element in the list starting with hydrogen as unity. It is unlikely that any new elements will be discovered between hydrogen and aluminium, although if they were it would be necessary to alter the whole of the subsequent atomic numbers to correspond. For X-radiations of the other series, the square-roots of the frequencies are not proportional to integers even, although the differences are nearly integral for successive elements in the periodic table. The actual numbers in the figure, 92 for uranium, for example, are derived from the assumption that the atomic number of aluminium is 13, but it is well to remember that, although relatively to one another based on experimental evidence, the absolute value is to some extent arbitrary.

T

[blocks in formation]

[At either Uranium-I or Uranium-II the series branches, and 8%
of the total number of atoms disintegrating, follow

the branch Actinium series.]

Uranium-I
Uranium-X,
Uranium-X
Uranium-11
Ionium
Radium
Ra. Emạnation
Radium-A
Radium-B
Radium-C

238 234 234 234 230 226 222 218 214 214

8200 8 8 8 8 8 :

8,000,000,000 years

35.5 days

1•65 minutes
3,000,000 years (?)
100,000 years

2440 years
5.55 days
4.3 minutes
38.5 minutes
28.1 minutes

Uranium
Thorium
Ekatantalum
Uranium
Thorium
Radium
Emanation
Polonium
Lead
Bismuth

[blocks in formation]

B

B (99.97%)

Uranium-Y
Ekatantalum
Actinium
Radioactinium
Actinium-X
Ac. Emanation
Actinium-A
Actinium-B
Actinium-C
· Actinium-D
End Product

a

2.2 days

Thorium 1000 to 10,000 years (?) Ekatantalum ?

Actinium 28.1 days

Thorium
16.4 days

Radium
5.6 seconds

Emanation
0·003 second

Polonium
52•1 minutes Lead
3.1 minutes Bismuth
6.83 minutes Thallium

Lead

Radium-C
Radium-D
Radium-E
Radium-F

(Polonium) End Product

214 210 210 210

1/1,000,oooth sec. (?)

24 years 7.2 days 196 days

Polonium
Lead
Bismuth
Polonium

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a

[blocks in formation]

(either
206

or
210]

II. THORIUM SERIES.

[At Radium-C, 0.03% of the atoms follow the branch series. ]

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[At Thorium-C, 35% of the atoms follow the branch series.] Thorium-C

212

87 minutes Lead

(35%) Thorium-D

208 B 4.5 minutes

Thallium End Product 208

Lead

Thorium-C End Product

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212 208

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(Face page 134. Sequence of Changes of Uranium (U) and Thorium (Th) into various

Isotopes of Lead (Pb.)

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