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“ characteristic radiations’ were studied and their wave-lengths measured 
_ by Moseley working in the laboratory of Sir Ernest Rutherfordin Manchester, 
Moseley showed that there is a steady shortening of wave-length as we pass 
from one element to the next higher in the periodic table, and that according — 
_to a remarkably simple law (v. Fig. 4), the differences of the square roots 
of the characteristic frequencies remain constant between each successive 
pair of elements. From this law it is easy to show that between the first 
element of the table—Hydrogen—and the last—Uranium—there remain yet 
undiscovered five elements and five only. Moseley also confirmed Barkla’s 
discovery that elements of high atomic weight, at least, possess not one only 
but two characteristic radiations (the so-called K and L radiations). Later 
FIG. 4,—Moseley’s spectra of X-rays (principal K and L lines),- 
from different metals, showing progressive change o' 
wave length. 
* 
investigators have added a third, the M-radiation. These three types of — 
X-radiations show a progressively increasing wave-length, and consequently 
decreasing power of penetration. Thus the wave-lengths of the strongest 
line in the K-radiation from tungsten is .212 Angstrom (one Angstrom = — 
one ten-thousandth of a metre; the wave-length of yellow sodium light is 
5,890 Angstroms) ; that of the L-radiation 1.67 Angstroms ; that of the 
M-radiation 7.01 Angstroms. The wave-lengths for other elements vary cor- 
respondingly. The K, L, and M radiations are not strictly monochromatic, — 
53° 
