In addition, Moseley's laws allow us to calculate very accurately the wave-lengths of the X-ray spectral lines for any element in the periodic table, if those of the elements in its neighbourhood ...

Studying the line spectra produced by hot gases and absorbed by cooler gases allows us to identify the elements in stars. When a gas is very hot, it doesn’t emit all wavelengths of light.

claimed to use highly accurate X-ray measurements and that this enabled him to record several spectral lines with precisely the frequencies expected of element 87, which he promptly named moldavium.