X rays and crystal structure . in atomic weight. Soddy haspointed out that radium B, actinium B, thorium B,and radium D are all isotopes of lead, showingchemical properties identical with those of lead, andinseparable from lead by chemical means. This isin spite of the fact that their atomic weights differfrom each other ; that of radium B is 214 nearly, ofradium D 210, of thorium B 212 and of lead which is common to them all is—adoptingRutherfords idea as to the structure of the atom—the positive nuclear charge. Considering the closerelation which, following the same theory, is sup
X rays and crystal structure . in atomic weight. Soddy haspointed out that radium B, actinium B, thorium B,and radium D are all isotopes of lead, showingchemical properties identical with those of lead, andinseparable from lead by chemical means. This isin spite of the fact that their atomic weights differfrom each other ; that of radium B is 214 nearly, ofradium D 210, of thorium B 212 and of lead which is common to them all is—adoptingRutherfords idea as to the structure of the atom—the positive nuclear charge. Considering the closerelation which, following the same theory, is sup- * Phil. Mag. 1914. 86 X-RAY SPECTRA posed to exist between the wave length of the X-rayand the nuclear charge, it is to be expected thatisotopes should yield the same X-ray fact that they are now found to do so is astriking confirmation of theory. The wave lengthswhich they emit belong to the L series of Barkla. rrLncipal (modiurn LintDiaTnoTuci. (n i). ?? and ^f^i-\<i Oxd-tr^conipQ-Ted. UHltcL is. 8° 25 It is to be observed, however, that in all thesecomparisons we have picked out the importantlines from the various spectra, and we are still inignorance as to how to place the rest. We do notunderstand in the least the mechanism through whichit is brought about that so many of the elements X-RAY SPECTRA 87 emit two-line spectra, and that in others there is astill more complex arrangement of five lines atleast. Even the principal line of ?^he two-line spec-trum is really a very close doublet: the wavelengths of the rhodium doublet being, for example, X io~^ and x Io~^ We are not yet ableto give any explanation whatever of these departuresfrom simplicity. The double nature of the rhodium line is wellshown in the foregoing figure (Fig. 25), whichcompares the first and third order reflections in thecleavage plane of the diamond. In the first orderthe double nature is indicated only, but in the thirdthe doublet is completely resolved. The treb
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