The Discovery of Helium

Posted by jns on May 8, 2005

“Observations of the 1868 [solar] eclipse led to the discovery of a bright yellow emission line in the spectrum of the [sun's] chromosphere, which is normally not observable except during a few seconds just before and just following totality [in a solar eclipse]. What happened next is nicely described by C.A. Young in the 1895 edition of his book The Sun:

The famout D₃ line was first seen in 1868, when the spectroscope was for the first time directed upon a solar eclipse. Most observers supposed it to be the D line of sodium, but P.J.C. Janssen noted its non-coincidence; and very soon, when Lockyer and Frankland took up the study of the chromosphere spectrum, they found that the line could not be ascribed to hydrogen or to any then known terrestrial element. As a matter of convenient reference Frankland proposed for the unknown substance the provisional name of “helium” [after the Greek name for the sun, "helios"] …
Naturally there has been much earnest searching after the hypothetical element, but until very recently wholly without success ….
The matter remained a mystery until April, 1895, when Dr. Ramsey, who was Lord Rayleigh’s chemical collaborator in the discovery of argon, in examining the gas liberated by heating a specimen of Norwegian cleveite, found in its spectrum the D₃ line, conspicuous and indubitable … Cleveite is a species of uraninite or pitch blende, and it soon appeared that helium could be obtained from nearly all the uranium minerals.

“As we now know, the connection between uranium and helium is that radioactive decay of uranium involves what were at that time called alpha particles, which are helium nuclei. These nuclei pick up electrons to become atoms of helium, which can become trapped in uranium-rich rocks, to be released when the rocks are heated.”
[Nearest Star: The Surprising Science of Our Sun, Leon Golub and Jay M. Pasachoff (Harvard University Press, Cambridge, MA, 2001) pp. 141--142.]