In 1894, Lord Rayleigh reported that the mass of chemically prepared nitrogen was different from that of nitrogen extracted from the atmosphere, as shown in Tables 1 and 2. Latter, this difference was attributed to the presence of argon in atmospheric nitrogen. The masses of gases were mearsured by using a glass vessel with a known volume under atmospheric pressure (1.013xx10^(5)Pa). |{:("From nitric oxide",,2.3001 g),("From nitrous oxide",,2.2990 g),("From amonium nitrite purified at a red heat",,2.2987 g),("From urea",,2.2985 g),("From ammonium nitrite purified in the cold",,2.2987 g),("Mean",,2.2990 g):}| |{:(O_(2) "was removed by hot copper" (1892),,2.3103 g),(O_(2) "was removed by hot iron" (1893),,2.3100 g),(O_(2) "was removed by ferrous hydrate (1894),2.3102 g,),(Mean,,2.3102 g):}| Ramsay and cleve discovered helium in cleveite (a mineral consiting of uranium oxide and oxides of lead, thorium, and rare earths, an impure variety of uraninite) independently and virtually simultaneously in 1895. The gas extracted from the rock showed a unique spectroscopic line at around 588 nm (indicated by D3 in Figure 1), which was fist observed in the spectrum of solar prominence during a total eclipse in 1868, near the well-known D_(1) and D_(2) lines of sodium. ul(Calculate) the energy E[J] of a photon with the wavelength of the D_(3) line of helium shown in Figure 1 Figure 2. Energy diagram of atomic orbitals of helium when an electron resides in the 1s orbital. Figure 2 shows an energy diagram of the atomic orbitals of helium. The arrows indicate the ''allowed'' transitions according to the spectroscopic principle.